If it were 100% wide on 4K, how would your eyes scan the text? It's useful for illustrations (graphics), but I haven't met a person yet who can read wide text as efficiently as one or more narrow columns.
That way your primary monitor can still be used to watch media and play games in the "standard" widescreen format, but your code and browser can readily be viewed on the sides.
IPS panels should work without a problem in both landscape and portrait mode.