If you planning to buy something in the 40 inches range, go with 4k. Full HD (1080p) in this size will give you pixels view. I'm with a 40" samsung television full HD and is not that sharp in comparison with lower screen sizes in the same screen resolution. This don't bother me, I'm happy with the big size screen, but consider this to you.
Other thing, try to find a freesync monitor.