The image is a lot clearer. 1080p feels blurry and smudged after playing on a 4k monitor. Also there is no aliasing (jagged white edges)in 4k, and I even have AA disabled. Level of detail will mostly be matter of the textures, which are the same for 4k or 1080p. It also feels less claustrophobic. You can adjust your fov in the settings.
I would suggest a freesync or gsync monitor if stuttery framerates bother you. Hard to say about the headaches without knowing why you get them.