Nah, skip PS4 and make it launch title for PS5.

a new ip with a brand new engine takes usually 5 to 6 years to develop, creating new assets and artwork takes the longest time, but with an engine and story in place a sequel will take less time, also god of war took 4 years to make, SSM cancelled a brand new IP in 2014 and cory was asked to jump start the project as he was still figuring out the story. so i wont be surprised if we see sequel in 2 years time.

Cory rejoined SSM at the end of 2013, game started development after feb 2014 so 4 years..

3 years at the least. It will not be multiplatform. They want to do the hard generational cutoff and developers like to focus on one set of hardware (PS4 and PS4 pro are almost identical hardware wise just some arbitrary number differences, can mostly push the same software tech) and PS5 is going to be vastly different than base PS4.
We will finally get the hundreds of enemies on screen next gen for god of war but you could argue it doesn't matter.

The big question is regarding Sony having the PS5 out before the holiday season of 2020. They're gonna be under pressure from the Switch over the next 2 years and we don't know if Microsoft have any plans for a new platform. They *could* push the PS4 for another few years with well timed pricecuts and hardware revisions though.

Personally, I wouldn't be surprised to see them try push the PS5 out to the 2020 holiday season. If that happens then it would be perfect for SSM to make the next game the swan-song for PS4 in the early Summer of 2020 and hit a remaster on PS5 12 months later.

Um, didn't God of War 2 come out on PS2 in 2007?

https://www.psu.com/news/god-of-war-ps4-has-been-in-development-for-3-years/

According to this article SSM stated that the game was in development for 3 years on November 2016. So their might have been some pre work done on God of War before Corey got involved. Although it could very well be just building the engine.

Anywho if they want to release God of War on PS5 they probably need to update the engine again and God of War games are generally quite big, especially this one. So I really doubt the new god of war game can be developed in 2 years. The Dev team was approximately 3 times the size of GOW2 (ps2) is team and counted 270 heads. So I doubt they can speed the process by increasing the size of the team.

Imagine 2021 having both a new God of War and  a new Horizon Zero Dawn. That's a very good way to start the gen. If the PS5 releases in Q4 2020 that is. For now the PS4 seems to coexist rather well next to the Switch. Seems like neither really affects the other at this point. Sony can still drop the price to $200 permanently and sell a shitload of units.

Maybe 5 years is pessimistic, but I still think it won't be 2 to 3 years. Will be happy if they make it within 2 years with the same quality of course, but take for example MS and their premier franchise with Halo. Halo 5 is already out, and by the time Halo 6 comes out it will have been at least 3 years if not a bit more, putting it as 3-4 years for a franchise that MS would want an entry out frequently.

The sequel to Horizon will likely be out for the holidays next year and we'll probably get a trailer for it sometime late this year. No way at all they take 4 years to make a new one. Guerrilla Games is good in terms of output, 2 years between Killzone 3 on PS3 and Shadowfall on PS4. They built the Decima engine for Horizon, so expect it to see some more use from them on PS4.