We suspect (not proven) because its called 4Pro, that when PS5 comes out, the 4Pro will not be able to play PS5 games. As in the expectation is that 5 will be a new generation. Though we expect ps5 can play games backwards compatible though.
That is an option both ms and sony have if the benefit outweights the disadvantage.
Sony has interesting option in breaking compatibility. All ps4 pro games also come out for ps4. PS5 could be ps4 compatible/emulate ps4 versions of games. That way even if ps4 pro wasn't compatible all the users library would still carry over to ps5.