The whole idea behind 4k is that you can feed it 1080p content and it will be displayed no worse than it would be on native 1080p display. In other words it allows easily swap between having conventional 24" 1080p performance or full 4k resolution, and this swapping can be relatively easily be done on a per-application basis, or possibly even more granually. Imagine eg having a WebGL context being pixel-doubled while the text on the same page being rendered at full resolution.

In comparison, in a 2560x1600 24" monitor you'd get either quite big/ugly double-pixels, or scaling artifacts of non-integer multiple scaling.