I wanted to have 16 windows open at once so I could see all the spreadsheets, text files, folders, pictures relevant to the legal matters I was working on. The 3840 x 2160 pixels lets me do it, but there are drawbacks and limitations:
1. Only the windows in the center of the screen are easy to perceive. I have to turn my neck or my eyes to bring the other windows into my attention, or sit further away from the whole screen, as if I had a smaller monitor.
2. The Mac menu is all the way up the top left. That makes using any app in any window that isn't top left difficult because I have to keep sending the mouse away long distance.
A friend got something instead which is better: Two very wide and not very tall monitors, which he arranges vertically stacked. What makes this so successful is that each monitor has its own menu.
I wish I could accomplish the same thing by telling OSX to split the screen horizontally across the middle so I had two menu's too.