You need to produce it first. If they planned 2 millions sales, they won't be able to sell 4 millions. Maybe 3M if they planned some additionnal stocks...

No it didn't, it sold it in much less time.

PS4 sold 2.1 Million Units with:

15 Days in USA

2 Days in EU

0 Days in JPN

That's very different to a global launch, with 2 million for 27 days in all regions.

2 Million for a global launch month is not good.

You need to produce it first. If they planned 2 millions sales, they won't be able to sell 4 millions. Maybe 3M if they planned some additionnal stocks...

Are you telling me you seriously think they'd only produce 2m units come March 3 and have nothing else because if so you're kidding yourself, companies normally produce additional stock they may very well already have 3-4m units in total come launch while shipping only 2m initially and the additional 2m for the launch window and following month. 

And when they say that they'll ship 2M for launch, even if the launch is successful, 2 more millions can't magically appear out of nowhere. It takes a lot of time for production and shipping. Production is carefully planned long before launch, and the shipping announcement depends on the production plan.

So, if they made a production plan for 2M at launch and then an average pace of production, they can't push 2 more millions just like that on short notice. You said that yourself: an initial shipment and then an already planned number for the launch window and the following months. Even if the console is a lot more successful than expected, they can't double or really speed up this plan. Your "2M for the launch window and the following month" will arrive as expected, during the launch window and the following month. It won't appear magically a few hours/days after launch on the step of every out-of-stock shop. Like someone else said, they still can ask to speed the production up now, but it will be too late if they want to react on launch day.

So your initial comment of "If they sell out day one another shipment will be sent that week" is still wrong, or at least exaggerated. The next shipment will be planned before that and won't really depend on the launch day success. If they sell out day one, there will be a shortage while they ship the second shipment more or less as already planned and prepare the third. These following shipments won't be that much faster or bigger than already planned. They won't be able to do all of this in a day because they saw that the console is sold out, you can't speed the process up at will. You can stop it if it doesn't sell, but it's really difficult to do the opposite. 

If they were able to do that, the NES mini launch wouldn't have been this awful.

the weirdest one is people saying a 2m launch month in March wouldn't be good

Do you know what a forecast is first of all? It's not a set in stone figure it's an expected amount that is subject to change it's nothing more than speculation like anything else to do with investors and the markets, it also doesn't take a lot of time to produce a console especially one where the chipset is based of a chip another company has had lying around not being used.

When Nintendo gave the forecast they already planned to produce more than the figure they gave that's how businesses operate it's nothing new or out of the ordinary they most likely already have 3-4m units ready as that would cover the immediate period and upfront demand, it's obvious to anyone with common sense 2m is going to sell out fast so they'll need to restock fast which is why excess is made.

NES mini is different from a new platform that's in the long term plans of a company, not even in the same ball park.

Yeah, they surely already have millions of additional stock... You don't know anything about stockage costs and the fact that you never produce a lot more than what you expect to sell, do you? 

But hey, who am I to talk about it. You're right, I surely don't know how a forecast and a production plan work... I'm only a production and logistics engineer after all! 

You do realize the PS4 sold over 1M in just a day in just the US and Canada at launch, right? 2M WW at launch is to be expected if you have the units available. Well, unless your product flops hard. I don't think anyone thought it would flop hard. The worst case scenario I've seen is it doing well at launch and then fizzling out slowly, like the Wii U and 3DS, to a lesser extent, did. In fact, all the arguments I have heard from Nintendo fans reminds of those launches a lot. Still, I don't see it doing as badly as Wii U, but I also don't see it pulling 3DS numbers.

You do realize the PS4 sold over 1M in just a day in just the US and Canada at launch, right? 2M WW at launch is to be expected if you have the units available. Well, unless your product flops hard. I don't think anyone thought it would flop hard. The worst case scenario I've seen is it doing well at launch and then fizzling out slowly, like the Wii U and 3DS, to a lesser extent, did. In fact, all the arguments I have heard from Nintendo fans reminds of those launches a lot. Still, I don't see it doing as badly as Wii U, but I also don't see it pulling 3DS numbers.