Unless you're going to be hooking up to a 4k tv with the duplicate screens setting I would recommend not getting 4k. I use one to train people at work for this reason but its incredibly hard to use at native resolution. It is as PolyHertz says an insane amount of DPI for something so small. I saw a 4K 32" screen the other…
Yes, at 32 inchs 4K will look great, but at 15 inch it'll cause eye strain. The size of the screen dictates what resolution makes the most sense to run it at, with the higher the resolution the larger you want it and vise versa. The eye strain you mentioned when looking at a 1080p set was likely due to it being too big…