The PS5 will have most of the PS4 games playable and i'm sure that will come into play starting this fall.
Switch should have a 13+ million lead over the PS4 after the holidays, which is insane. I think the PS4 will die a little before 125 millions, could have done 130+ but sony doesn't seem like they want to push it no more. But eh, Nintendo has a habit of doing things like this so it's nothing new. DS and Gameboy could have done more but they cut their legs short.
Switch will blow past the 100 million mark and at least end up in the 120 million range i think.