Oh it will, it's a standard architecture. Unless of course they design it in a way that it won't.
No. That's not how any computing devices work. Sony can develop the PS5 to have extremely similar architecture to the PS4 and it still wouldn't automatically run PS4 games. It's like saying my desktop will run such and such a game at x settings - well only if you have Windows 10 installed. It's probably not going to run that game if you have linux of mac OS installed, or even Windows XP, is it? See the operating system needs to be able to read and interpret a PS4 game and run it correctly. The PS5's OS will have to be developed to do that, it won't just automatically run PS4 games if it's been programmed to run PS5 games.