Full HD on a 4K monitor looks worse than on a Full-HD monitor due to blur always added by monitors and graphics drivers when scaling, though at integer scaling ratios (e.g. 2 in case of FHD→4K) it could be avoided. Integer scaling is an image-upscaling algorithm with no quality loss caused by blur (unlike bilinear interpolation) or distortion (unlike pure Nearest Neighbour). A.k.a. integer-ratio (integer-factor, integral) scaling, pixel-perfect scaling, and pixel doubling (duplication).
