@shyhh If you don't know, the PS4 Pro had to have games patched or already set for the enhanced Pro benefits. If not, then the Pro itself ran exactly like a regular PS4. That wasn't just switching off half of the GPU but it also down-clocked the CPU and GPU to ensure that every 'non-enhanced' game ran on the Pro without any issues.

Months later, Sony did add in the option for a boost mode with a disclaimer that some games could experience issues. Boost mode still switched off half the GPU as before but kept the CPU and GPU clocked at the same speed as the full Pro. Boost mode essentially upped the GPU to 2.1Tflops from the 1.84Tflops the standard PS4 runs at.

There were a number of games though that exhibited issues, crashing for example as one of the worst cases because the games were optimised for the specific clock speeds of the Jaguar CPU and/or GPU - being faster, it messed with the tick rate of the games which caused problems.

For total backwards compatibility on the PS5 without causing issues with some games, Sony would have to ensure the PS5 can run exactly like a PS4 - or at least emulate that in software. If they also want the Pro enhanced games to run like they do on the Pro, then Sony would also need to ensure the hardware can run exactly like the Pro did or emulate it in software. If you also include the 'boost mode' as well, then you have 4 different configurations that the hardware must be able to do - at least 3 if you discount the boost mode.

Sony could do something similar to the Pro - ie have a GPU that can switch off sections depending on the game - PS4, Pro enhanced or PS5. They could patch (or allow devs to) in PS5 settings for existing games - change U4 to 4k/60 and tweak any visual settings that maybe can be pushed up to ultra for example.They obviously have been running PS4 games (Spider-Man, GTSport) and the most 'logical' (in my mind) would be to patch these games to run on PS5 and take advantage of its hardware. The problem though is the numerous games that perhaps won't be patched and, to keep them available on PS5, the PS5 would need to run like a PS4 and Pro too - if it runs locally of course. Another option could be streaming so Sony wouldn't need to worry about building the hardware to run like a PS4 and a Pro because the games are running in the cloud instead. It will be interesting to see what they do, whether its full or not and the way BC is implemented...