@Daz:
Question: If a game exists both on the ps3 and ps4 store, and I owned it on ps3, would I be able to re-download it on PS4?
It varies wildly and depends on the game. By mostly, the answer is no, because the PS3 and PS4 architectures were so different the games mostly aren't compatible unless the company put in the work to port it specifically and so its treated as a different game.
And because devs want all your money so they want you to buy it a second time. But there are exceptions.
For instance, Journey you can get on the PS3 and PS4 with the same purchase, and I imagine it'll port to PS5 as well. (Though they actually made that a free quarantine game so) Different trophy set.
Shadow of Colossus you could get remastered on PS3, but then on PS4 they did a complete HD remake from scratch, and its two seperate games as two seperate purchases.
Okami you could buy on the PS3, and on the PS4, but those are actually separate purchases for whatever reason… but they share the same trophies.
It gets even weirder with PS1 and PS2 games because you could buy those on the store and its anyone's guess what given system they'll run on, but the PS3 and PS4 ports are different and likely seperate purchases..
Suikoden, which was a PS1 game, you could buy on the PS3 and that same purchase could be played on the PSP (which was PSP's main selling point)... but you can NOT get it on the Vita or PS4. Suikoden 3 does run on Vita, but not on PS4.