^thanks master! i was in the impression that the 720p to 1080p and 1080p to 4k is the same ratio
That's part of the beauty of UHD resolution. Properly done integer conversions do not cause any loss of data.
480p OS (432p) to UHD: 5x linear, 25x pixel count
HD (720p) to UHD: 3x linear, 9x pixel count
FHD (1080p) to UHD: 2x linear, 4x pixel count
480p OS (432p) to 1080p: 2.5x linear, 6.25x pixel count
HD (720p) to 1080p: 1.5x linear, 2.25x pixel count
UHD (2160p) to 1080p: 0.5x linear, 0.25x pixel count