Im trying not to be too harsh here, but surely testing an 8k map yourself would have been quicker to do instead of asking on the forum and waitng for an answer. It will basically all depend on what your system will handle. You could also uprez the map in PS if you can only do 4k as an outputed snapshot.
Bare this in mind as I think it escapes alot of people. Do the map at the highest you can go, lets say 8k for arguements sake. You can actually down size it to 4k and it will still retain a fair amount of the original detail. Its the same as going from 4k to 512, you'd be surprised at the detail you keep. As long as you keep the hi res for doing the work....
Also think how close the camera is going to be for your final shot and is it for animation?? Motion blur is your friend sometimes
when I did the textures for the spacecraft on the Stephen Hawking show they averaged around 4k with the odd 8k thrown in. Dont forget too that HD is only 1080dp so......these maps looked fine even with the ship cruising by the camera. turn off your quadratic filter too when you load the maps in, Maya always filters the textures as a default so you end up with pointless blurring when you render it out.