Sony has a heterogeneous history of backward compatibility, from concept adoption to its recent rejection as unused features. Now we've seen the first details for PlayStation 5 and include backward compatibility with PS4.
Mark Cerny, who was the chief architect of the PS4 system, says the new generation of PlayStation (which is not yet technically named "PlayStation 5") uses a similar architecture to its predecessor, allowing this feature. It will also use physical disks and Cerny expects some games to be released in both consoles.
However, PS4 games will run a little faster on the PS5 by incorporating an SSD. As an example, Cerny compared the load times for fast travel in Spider-Man. The PS4 Pro took about 15 seconds, but the same sequence lasted less than one second on the PS5 dev. So if you upgrade your hardware, you'll probably have a nicer smoother experience in some ways.
We still don't know exactly how far a new generation of PlayStation could be. Last year, PlayStation chief John Kodera suggested that the new system is still three years old. That would mean it was two more years away, so either Sony was preparing a long ramp to run, or the timetable was considerably accelerated.