With Polyphony mentioning that GT Sport is designed to scale up easily for whatever console is coming in the future from PS, I would have to think that ND, as well as most others, are aware of the basic info they need for the next console.
If it ends up being the PS5, and it's BC, then PS and ND should wait to launch it when PS5 launches. The game can launch for PS4 at the same time, but if they launch TLOU2 say March 2019, and launch PS5 November 2019, many people will just continue to use their PS4 copy whenever they eventually buy the PS5.
If PS5 is not BC, then PS and ND might as well launch TLOU2 on PS4, then remaster it for PS5. This worked wonders for TLOU Remastered and GTAV. They could also choose to make the PS5 $499, since they seem to be torn about the next gen leap vs affordable price, and bundle TLOU2 with it as an exclusive launch title. That should be enough to entice many gamers into next gen early on.

I’ve been thinking this for some time. I can see Sony recognizing the success Nintendo had with their launch of BotW, and how both it and MK8D played a vital role in bolstering Switch’s initial sales until Switch-only games released. Honestly, it’s a pretty smart so as to not rush out PS5-exclusive software, especially if they can good late 8th gen games of high quality out there to do it. 

That leaves only one question, which is when will we see PS5? With Pro out on the market, a PS5 doesn’t seem nearly as urgent. I think even late 2019 might be too soon. 

man i wonder if ill be tired of my ps4 by 2020

Naughty Dog has been working on two games at a time for years now. TLOU2 in late 2018 wouldn't surprise me.

2021 PS5
2019 TLOU2

Nope.

First off if using how long it took to make uncharted as a yardstick then you are already making a mistake. Every game ND makes now using the same engine they used to make U4 will be easier to make and take less time.

Secondly the PS5 most likely won't be coming out till 2020 at the earliest.

Lastly, Usually it would be a smaller team working on the U4 DLC..... while the rest of the team continues work on TLoU2

Nah, unless PS5 release in 2019, because I doubt they would unveil a game in 2016 and release it in 2020 as a cross gen title. This would be bad PR if you ask me.

Perhaps not cross-launch, but I could see it releasing on the PS5 later, more polish added to it.

I think ps5 games and ps4 games will be just like ps4pro and ps4 games. They will work just like a minimum system requirement. The minimum requirement of a 2020 game could be ps5 and for other 2020 game could be ps4.

2021?! Never ever. 2020 is possible still but I expect 2019. Makes perfect sense. 3 years after the Pro, 6 years after PS4. 7 years of last gas was already unusually long.