Hmmm! but if they upped the resolution to 1080P then the already very low & unstable FPS would be worse. I am curious though, could they not have off-set some of the CPU tasks to the GPU on PS4 ?
It's possible.
It's also possible that they could have moved the bulk of the crowd AI and tracking to the cloud for X1 (which wouldn't have cost them anything), and then the performance would have been substantially better on X1, even if they were able to move some of the CPU processes to the gpu On PS4.