People thought 2017 would be down on the impressive 2016. But you have to remember that while,  yes ps4 slim launched and pro launched late 2016, 2017 enjoys those benefits for the entire year and then likely gets the bonus of further cuts this holiday. 

Likewise if we see a $199 slim this holiday and a $299/349 Pro,  2018 will actually benefits massively from that too as it's baseline will be higher starting in January. 

I would be very impressed if it doesn't peak this year. Also, imo most RDR2 players are also GTAV players, so it won't be that big of a system seller.

There're many Rockstar fans on PC. Especially in all countries east of Germany and Xbox is non-existing there.

This is extreamly tough to say. There are a massive amount of variables, based on how the market reacts, and what Sony ends up doing.

As I said yesterday in the predict PS4 future sales thread, I see three likely possabilities going forward.

1) PS4 and PS4 Pro are the only two PS4 models Sony makes. PS5 arrives in 2019.

2) A PS4 Premium is added to the stable in 2018, along side a PS4 Super Slim and PS4 Pro Slim. PS5 Launches in 2020.

3) PS4 Premium + PS4 Super Slim + PS4 Pro Slim 2018, PS4 Latpot/Tablet & PS4 Portable/Phone 2019, PS5 in 2020, PS4 Set Top Box 2021, PS4 Mobile line upgrades 2022, PS5 Pro and Slim 2023, PS5 goes mobile 2026.

All three have diferent peaks.

1) I would say 2018 is the peak, and we end up getting solid PS5 rumors by the end of the year which slows down PS4 sales in 2019.

2) I see the peak in 2019, with sales again slowing in 2020 with the PS5 looming.

3) I could see sales peak in 2023, with the rumors of PS5 Phone coming.

I think 2017 will be the peak year. It the first full year of PS4 Pro. Not no mention it had better exclusive than 2018 will have. I don't think Spiderman is going to beat Gran Turismo. Neither will GOW beat Horizon (which is looking to be a 8m seller or something), etc. I feel third party will be slightly weaker next year as well. 

Regardless... I think 2018 will come close to 2017 and probably reach 16m or 17m.

I think this year will be the peak year, so ~18M sold through. We have GT Sport launching, which is Sony's largest exclusive. And we also have a $199 PS4, at least for the holidays, and a $349 Pro.

Next year, I think we will see sales start to slip as the gen starts to age. I'm guessing a full $199 PS4 and a $299 Pro will help it not be down by too much, so possibly ~16M sold through. And in 2019, we will have a few exclusives help it not drop too much, again, so maybe ~12M-14M. At this point, I think Sony is going to sense the writing on the wall and launch the PS5 late 2019, coupled with a cut to $149 for the PS4. The next year should be down, again. So, ~8M-10M. In 2021-2020, I think we are going to see the last few sales of the PS4. So, ~6M-8M for both.

So, in the end, it'll probably sell ~115M-120M. Possibly more.

2018 for me, it will have so many huge games and will likely get another price cut. That said I'm not gonna be surprised if it does peak this year

If GTA VI is FC and can be played on PS5, then there won't be as much of a push to make sure it lands during this gen. If for whatever reason its not FC, then getting it out on PS4, a year or two before PS5 releases, will be a goal, so they can re-release it on PS5, just like GTA V on PS3 and PS4.

If TLoU2 manages to launch in 2018, along with (maybe?) Death Stranding, I can actually see 2018 being PS4's peak year. I'm willing to bet 2017 will be even better than 2016, with BF2, Destiny 2, and the most popular CoD in year, all releasing this holiday.

2018. 199€ PS4 Slim with all the games will be better than this year.