Playstation 4 will continue to sell in emerging markets and through low cost bundles. A new Ratchet & Clank hitting a 7 year old console does nothing to move additional units but will do a lot for a console that's only going to have ~20 games available at launch not counting BC.
The PS5 is going to sell itself the first year just for the novelty of being the new PlayStation. I doubt Sony truly sees Ratchet being exclusive to the PS5 as a pivotal release during that time.
You also have to remember that Sony wants to make money on their games and they are cutting off a huge install base with the PS4 for a game that would do rather well at this point. Games like Ratchet have historically proven to do better after a few years when younger demographics start adapting to the new platforms and the prices go down.
I mean, Knack was a PS4 launch game but one that was more of a tech demo and felt shoddy as a PS4 gen game. The new Ratchet no doubt has a much bigger budget and we will have to see if Sony thinks it can do well as a PS5 exclusive game or if it needs a PS4 version too. Right now it just feels like it can go either way.