Bit of an old post, but I run my 60" 4K TV at 1080p, but I do sit at TV distances. If you are sitting very close then I imagine you are seeing groups of four pixels to make up one. Since the pixels aren't in a perfect square grid you might see the slight artifacting as a result.
If the TV has any upscaling or smoothing settings, you could try disabling them. If it has a PC mode, try that as well.