No one knows at this point. My guess is that unless Sony gets too arrogant again, or if PS5 is a dramatically different beast compared to PS4, it will at least support BC with PS4 titles. The big reason why PS4 didn't support PS3 titles was due to the PS3 hardware being a clusterf--- and this difficult to emulate. Pretty much all current gen remasters of last gen titles that lacked a PC port (such as the Devil May Cry HD Collection) are based on the 360 ports for this very reason. PS4 on the other hand has x86 based hardware (as is Xbox One) bridging the gap between console and PC. That's going to be the new model going forward for both Sony and Microsoft I'm sure. Easier to port games between all those three formats.