That doesn't really make sense. The reason they would run without poking and prodding is because they use different CPU and GPU architecture. Yes it's x86 based stuff but the language and function calls in the games are different than PC and they are also generally different between console generations.
The PS4 ran on GCN and games would be made for it very specifically... The PS5 is presumably RDNA2 which is completely different but more importantly so is Sony's sdk and the ps5 compiler. It shouldn't even really understand a significant amount of the code from a PS4 game.
I dunno... Guess we will see but I prefer Microsoft's method. Of course between series x and Xbox one it's 100% compatibility. With Sony it's apparently it will run but probably badly? I think I'd find that more obnoxious than anything. Of course I only really own system exclusives on the PlayStation side of things so I imagine those will work so it won't matter much to me in the end.
Probably be able to emulate them before long anyways...
I was heavily leaning towards getting the series X before getting a PS5, but with the PS5 having more exclusives that I actually want to play, the ability to run any PS4 or even most PS4 games is making me reconsider. I'm hoping it will enhance the PS4 games, in which case I'd just get an external SSD for my One X and wait a year before buying the series X since I don't plan on buying both at launch, but I definitely want to get one of them at launch and will wait to buy thee other one until something comes out that I want and can't buy on the other one. The new fable is the only game on series x I want at the moment and hopefully that also comes to the one x.