The games can't be that graphically demanding if a 1080 is rendering them at 90 frames per second. The ps4 has no chance in hell.
Depends; and again, it's quite subjective.... a lot of it has to do with qualitative judgements about the end product and that largely depends on the medium it's being viewed on.
That is to say, you'll find different approaches to improving overall visual quality, qualitatively, between games that are VR-based and those that are generally ports/ Most game designers would probably choose not to use many of the more intensive shaders or textures in a VR game if there was not the same degree of qualitative improvement experienced by the end-user, instead choosing to spend processing resources on different improvements.
Many gamers have to choose for themselves what settings to enable, which to turn off, or what optimized profiles work best for them. By default for example, IIRC the Rift will try to use interpolation to get to 90 FPS (not requiring the game engine to do it through the standard render pipeline); so this means the intermediate frames are interpolated. So instead of needing to render at 90 FPS, the system can do 45 FPS and get by. For most users, this is okay; however, for many this might cause motion sickness.
For users who aren't satisfied or comfortable with interpolated frames again, subjectively, would prefer to run settings turned down a bit to achieve a native 90 FPS; which, a 1080 can do. Most often things like anti-aliasing and SSAO; for example, a 1080 can run GTA V @ 4k or near-4k at an average of 90 FPS on relatively high (not fully maxed) settings; whereas, if the settings were fully maxed out, you'd be looking at below-60 FPS performance... The question then becomes, are "max" settings visually different enough to warrant using them if near-max settings gets you 90 FPS?
So it's not entirely straight-forward, and one can't really extrapolate from either the experience they get on their monitor, or from benchmarks of graphics cards on "Ultra" settings since the qualitative experience in VR is vastly different.