The hardware is the limit here and not the unity engine, if you have quad gtx1080 you could "make" 8k resolution AAA games.

Short answer, 4K refers to display resolution. You would need a 4K Monitor or TV. However if you're asking if you can use 4,096x4096 textures? yes although very slow, make sure to click on your texture and in the max size, set 4,096 and uncheck Generate Mip Maps.