Well then you would need four 2048² skins which results in the same amount of memory and all graphics cards should be able to handle 4096² textures.

There is a reason why you cannot change the UV layout of a terrain, simply because that is not how you texture it. You should use a multitexture effect/ shader instead and, depending on the shader, you probably only need let's say 4 256x256 textures (even one would be enough) and one blend map to make it look sharp.