it is not the number of year that counts but the number of consoles that make the install base
The install base doesn't really matter though if most of those old consoles have since been shelved or are playing the same five games since 2019...
Consoles as they go on plateau and then taper off in attach rate when new consoles come out and as some owners lose interest over time. Sales are usually bunched and not publicly reported in detail, but we know some details about sales trends for 2022's Horizon Forbidden West which point to a 70/30 PS5 to PS4 ratio in its launch (despite the install base having lapped up the original Horizon on PS4, plus the the PS4 version of H:FW was upgradeable to PS5 at $10 less.) The resulting DLC for Forbidden West went exclusively to PS5 (which added some cool features and graphical advantages for flight, but it's debatable if a version of Burning Shores could somehow have been cut down if they really, really wanted to.) Warner Bros also dropped past-gen for most of its new games, even though some of them also add Switch versions where it makes business sense.
It's not necessarily a factor of power which would push GTA6 to current-gen. Yes, GTA6 will do some cool new stuff, and by now there are technologies mature enough to put in which wouldn't scale down easily if at all, but it's not like the old days where the power of the box defined the capabilities of the game. It could ignore PS4/One and still come to Switch and Mobile if they really wanted to. Rockstar is going to make the sensible choice all around, and at this point, there aren't a lot of reasons to have a team slog through the effort needed for releasing on the old consoles.
The leaks showed game running on PS4, but that couldn't mean to be anything. Worth noticing that Sony is crazy about selling more PS5, it could be because of the next GTA.
I don't think that's actually confirmed? There's was a connection drawn from what was seen of the UI in the leaked build, but PS5 dev kits apparently have the same UI as PS4.
Either way, in the early prototyping days, there are a lot of different builds of games before a fully integrated build can be tested with current features and functions. You can test scripting and other aspects of the project with what's available while waiting for the main to mix.
(Forza also leaked hinting at an Xbox One version, but the end product was Xbox Series-only, and also demanded a significant step up in minimum PC specs over the cross-gen FH5.)