A FHD screen is 1920x1080 = 2 073 600 pixels.
Running 3 screens will be 1920x1080 x 3 with a maximum resolution of 5760x1080 = 6 200 800 pixels
The 4k monitors have a resolution of 3,840×2160 = 7 516 800 pixels.
The 3 screen setup is about 2 million pixels shorter than the 4k, 2 073 600 to be exact, which is a full 1080p monitor short.
The 3 screen setup will probably wield better results than the 4k monitor but the 4k has the sharper image quality, although I have read that 1440p monitors are a step up from 1080p but there is not much of a jump to 4k.
/ snip benches
Looking at the above 4k seems to cost around 88fps vs. 1080p. Where it should be getting 38fps in 1080p. Computing is not that simple anymore, four times the screen size is not directly proportionate to the performance of your hardware. There are many factors to look at like the memory bandwidth, as the amount of memory for textures to load into.Rendering is done as a whole as far as I understand and not as 4 separate 1080p screens being done at once.