There is so much wrong with this post I don't even know where to begin. All you've demonstrated here is that you have absolutely no idea why video game consoles are made the way they are.

If there is a PS4K, It'll be a minimum spec bump at a minimum cost with a 4K bluray drive and an improved HDMI port to do 4K video output, and that is it. You will not see a easily noticable improvement to performance in video games, if any. There are many reasons for this:

• These are the most profitable years of a console. This is the point in the life where you introduce a cheaper sku at a cheaper consumer price with a higher margin. You do this because it's at this point in a console's life where the price gets below that pain threshold where you get most of your lifetime sales and thus most of your profits. Introducing a new higher spec console throws a monkey wrench into all of that.
• While it is true that introding a console with higher specs is cheaper than releasing a new console, it is still far far more expensive than producing an even cheaper version of your existing console. Not to mention you'll need to  develop and create new developer kits, test kits, and new developer tools to allow developers to take advantage of these new specs. This is one of the most expensive parts of console development.
• Video game developers, unless they are forced to, will most likely not support the additional specs because there the additional time and money associated with buying new dev kits and developer tools, and all of the additional QA work (which would be practically doubled to support it), not to mention engine development and dealing with specification-specific bugs is a handful. All for what? There is absolutely zero reason to expect supporting the improved spec will lead to additional  video game sales. So at the end of the day, the vast majority of games will not take advatage of the additional specifications, which has happened every single time a company has offered a console or add-on with a specification improvement mid-generation.
• If developers are forced to support all specs then those costs will be passed on to consumers. I'm sure PS4 owners won't be happy to pay $5-$10 more than Xbox One owners for the same game, especially if they don't own a PS4K.
• If they don't force developers to support all previous versions of the PS4, the new console will start to get some exlusive games, older PS4 owners will feel robbed because they bought the PS4 expecting it to play all modern PS4 games up until the PS5 came out (just ask owners of the non-new 3DS how much they enjoy playing Xenoblade Chronicles). This will piss off millions of people if they bought a console for christmas 2015 and now new games are coming out christmas 2017 (for example) that do not run on their PS4. This is the kind of anti-consumer decision that made many people buy a PS4 over an Xbox One.
• With a small existing consumer base (less than 10% of HDTV owners own 4KTVs, and a small percentage of those are gamers) and little games that will actually support the additional specifications, there's little reason to expect this will garner the tens of millions in sales that the high margin sku would otherwise generate

The idea might look good from a purely theoretically "best gaming experience" possible, but it falls far short when injected with reality.

Upgradable consoles are not the future of the console industry. All that does is confusion and frustrate the userbase. The reason consoles sell the way they do is because a PS4 owner can walk into the store, pick up any game with a PS4 logo on it, and know it will play in their console. It doesn't matter if they bought their PS4 in 2013, 2016, or 2018. They know that game will work. They never have to worry about compatibility. They never have to worry if their PS4 is going to run the game at a reasonable frame rate. They never have to worry if the game on their PS4 is going to be buggier than their friends. They never have to worry if their friend with a new PS4 has a competive advantage in online games compared to them. You know, all of the things you need to worry about if you own a PC.

See here's the thing. PC gaming is arguably cheaper if you're smart about it. People buy consoles not because they're cheaper initially, but mostly out of the convenience over PCs they offer. If you want a "better gaming experience" in terms of graphics and framerate? Buy a PC. Consoles have never been that solution. There's no reason to expect them to now.

Sony probably wants to push UHD blurays with the PS4K as an UHD bluray player. Maybe they also hope to push the 4K TVs as well.

4K wont be for gaming. The PS4 and XBO use a custom AMD APU chip which means the CPU and GPU is together on one chip (DIE). Can't see enough power for 4K gaming on an APU chip.

  This doesn't have to do with the NX, expect the NX will be also able to play UHD blurays. Then yes, this could be a reaction to the NX by Sony.

There is so much wrong with this post I don't even know where to begin. All you've demonstrated here is that you have absolutely no idea why video game consoles are made the way they are.

If there is a PS4K, It'll be a minimum spec bump at a minimum cost with a 4K bluray drive and an improved HDMI port to do 4K video output, and that is it. You will not see a easily noticable improvement to performance in video games, if any. There are many reasons for this:

• These are the most profitable years of a console. This is the point in the life where you introduce a cheaper sku at a cheaper consumer price with a higher margin. You do this because it's at this point in a console's life where the price gets below that pain threshold where you get most of your lifetime sales and thus most of your profits. Introducing a new higher spec console throws a monkey wrench into all of that.
• While it is true that introding a console with higher specs is cheaper than releasing a new console, it is still far far more expensive than producing an even cheaper version of your existing console. Not to mention you'll need to  develop and create new developer kits, test kits, and new developer tools to allow developers to take advantage of these new specs. This is one of the most expensive parts of console development.
• Video game developers, unless they are forced to, will most likely not support the additional specs because there the additional time and money associated with buying new dev kits and developer tools, and all of the additional QA work (which would be practically doubled to support it), not to mention engine development and dealing with specification-specific bugs is a handful. All for what? There is absolutely zero reason to expect supporting the improved spec will lead to additional  video game sales. So at the end of the day, the vast majority of games will not take advatage of the additional specifications, which has happened every single time a company has offered a console or add-on with a specification improvement mid-generation.
• If developers are forced to support all specs then those costs will be passed on to consumers. I'm sure PS4 owners won't be happy to pay $5-$10 more than Xbox One owners for the same game, especially if they don't own a PS4K.
• If they don't force developers to support all previous versions of the PS4, the new console will start to get some exlusive games, older PS4 owners will feel robbed because they bought the PS4 expecting it to play all modern PS4 games up until the PS5 came out (just ask owners of the non-new 3DS how much they enjoy playing Xenoblade Chronicles). This will piss off millions of people if they bought a console for christmas 2015 and now new games are coming out christmas 2017 (for example) that do not run on their PS4. This is the kind of anti-consumer decision that made many people buy a PS4 over an Xbox One.
• With a small existing consumer base (less than 10% of HDTV owners own 4KTVs, and a small percentage of those are gamers) and little games that will actually support the additional specifications, there's little reason to expect this will garner the tens of millions in sales that the high margin sku would otherwise generate

The idea might look good from a purely theoretically "best gaming experience" possible, but it falls far short when injected with reality.

Upgradable consoles are not the future of the console industry. All that does is confusion and frustrate the userbase. The reason consoles sell the way they do is because a PS4 owner can walk into the store, pick up any game with a PS4 logo on it, and know it will play in their console. It doesn't matter if they bought their PS4 in 2013, 2016, or 2018. They know that game will work. They never have to worry about compatibility. They never have to worry if their PS4 is going to run the game at a reasonable frame rate. They never have to worry if the game on their PS4 is going to be buggier than their friends. They never have to worry if their friend with a new PS4 has a competive advantage in online games compared to them. You know, all of the things you need to worry about if you own a PC.

See here's the thing. PC gaming is arguably cheaper if you're smart about it. People buy consoles not because they're cheaper initially, but mostly out of the convenience over PCs they offer. If you want a "better gaming experience" in terms of graphics and framerate? Buy a PC. Consoles have never been that solution. There's no reason to expect them to now.

If the rumours are true Sony develops a better version of the highly succesful PS4. But why? The system already sells great, where is the need of a newer model? Why not ride on the strong sales and harvest all those fat, greasy profits? Please don't say that console manufacturers start building new consoles after the last one launched. As far as it seems, this is no PS5. And 4K resolution alone? If that's the reason behind it, do you really think you would be willing to pay a whole new console? Is this maybe somehow linked to PSVR? And if so, how?

Or does Sony simply need to do something regarding the launch of NX? Is the PS4K simply a strategic response to Nintendos next console?

Yikes!! Please read the OP. You've simply derailed into why you think a PS4k would fail. The question is why would sony introduce it in the event it is real (several independent, reliable sources have said it is-Eurogamer being the most respectable). You rant implies that sony can see into the future and will only make decisions based on certified success when we've infact seem them and all manufacturers make bad decision which may have looked good on paper.

A few important points. 
-No one is expecting PS4k to have games as exclusives. 
-At no point is anyone assuming that there will not also be a PS4 slim. Sony will replace the current model with a slim for sure.

I have a full understanding of how both console cycles and profit margines work. The introduction of cheaper, slim "high margin" SKUs is also met with $50-100 price cuts. These pricecuts reduce the profit margins of the systems significantly,  close to where they were before. The real gain is partially in the higher sales which the lower price provides. This exact same mechanic can exist with a high end SKU. Use whatever economy allows for the cheaper slim model  (DIE Shrink?) to instead bump up the specs and skip the $50 price cut. Profit Margins remain the same but sales increase. Hell, if your gimmick is strong enough dare to increase the price. This can exist alongside a PS4 Slim just as Nintendo have the 2DS, 3DS XL and N3DS.

 http://www.eurogamer.net/articles/digitalfoundry-2016-in-theory-playstation-4k-is-real-but-why-is-sony-making-it

This high end SKU may not be mass market price but its a more compelling means to get people to double dip versus a slimmer console and it introduces a new market for people looking for a 4k device; soon to be many people- whether they're gamers is irrelevant when the PS4 is cheaper and more known then any 4k bluray player.

Regarding 4k TVs, how many people to do you think owned a 1080p TV in 2005 when PS3 was announced? 4k TVs are obviously not in tons of homes but they will be and sooner then you think. Their sales increased last year exponentially and they're only now hitting mass market prices. The industry forcast that 50% of US homes will own one by 2020. Literally the first things that pop up when you google 4k tv sales figures lol. Go into any store and they're the primary sets being pushed, soon they will replace all 1080p sets above a certain size. Sony could very easily swoop in and own that market. They'll miss out if they wait til 4k is already the standard.

Everything you said about it not being financially beneficial for Developers is fair, yet the exact same comments could have been made about the N3DS. Yet it exists, yet it was successfull. How many developers will support a more powerful SKU is still up in the air, but it will have first party support and some decent third party support that is a garuntee. Third parties even supported the PSmove for a while and that was a flop. Sony only need to get a few key thrid party games on board each year for it be a big selling point. Lets say PS4k launched last year, If sony could say Uncharted 4, The Devision, Final Fantasy XV, Fifa and Titanfall 2 would also have superior versions on PS4k that'd be more then enough to sell the system. Lastly lets not ignore the automated process of 4k upscalling to improve every single title for those who intend to get 4k tvs. Ultimately its irrelevant because that means we're assuming that sony can only make good decisions.

No. My post is why Sony isn't stupid enough to do what many people are assuming they will be.  What is the reason behind the PS4K that has enhanced hardware specs for games? There is none. That's my point.

- If no one is expecting the PS4k to have exclusives than the extra processing power will not be supported by third parties. There is literally no incentive for them to do so. See the N64 expansion pak, the PSP spec improvements, and the new 3DS. In all of those cases the vast majority of games that were released after that point did not take advantage of the hardware in any way. And you probably shouldn't act as if the new 3DS was successful, since the vast majority (over 95% of games since release ) don't take advantage of the added performance, and outside of the release window, has done absolutely nothing to inccrease 3DS sales. In fact regular 3DS outsell the new 3DS 2 or 3 to 1 to this day. In fact they sold more 3DSs at $200, then they sold new 3DSs at $200 over the same time frame. It appears people just don't see the added value, and it's not hard to see why. There is literally no incentive for third parties to support consoles like this. The Divison isn't selling any more copies if it's running at 60 fps as opposed to 30 fps. Sure some might buy it on PS4 instead of X1, but that's all the same money to the developer. That's an awful lot of effort to put in for literally zero reward. It is 100% wishful thinking to hope third parties would throw money away investing in developing games for these added specs for no reward.

- Doing a die shrink inviolves optimizing the process for that partucilar APU. Developiong a new more powerful APU would require doing almost all of the work of that die shrink all over again.  It's not that simple. It can take months of development to get acceptable yields. That's why when they develop a process of shrinking dies they don't just automatically start making all chips at that size. Also, you need to consider that a 4K APU would undoubtedly be more complex, use more power, and likely produce far more heat, and therefore, they may have to increase the die size, not decrease it, in order to produce it at acceptable yields.

- Yes when the PS3 came out, most people didn't have a 1080p TV, but most did have 720p HDTVs that would be a noticable improvement over the 480p output of the PS2. That's probably why most of the games at the time were optimized for 720p, and that's probably why 1080p only really became the objective with the PS4 and X1 and even then they're not really hitting the mark. Now you want to improve the specs and get them to render at 4 times the resolution they're struggling with now. Okay. How much do you want this PS4K to cost? Because if its going to play games at native 4K is going to cost a lot more than $400. The APU alone should cost at least that.

Besides, how would they be missing the 4K market at all if they make a PS4 slim with a 4K ready blu-ray player and HDMI port capable of playing 4K blu-rays, and maybe upscaling game video output to 4K, or maybe make some VR output a little smoother? Ohh right. They wouldn't. This is what Sony is probably making, in a slim for factor, with a higher profit margin, because that is what makes good business sense.