If you do some research, you’ll find that 4K is not actually a standard, it is an unofficial family of related standards, some of which havae 4096 pixels across, and some do not. UHD is among the “4K” standards available. Wikipedia has a good list of some of the 4K standards. UHD is the most convenient one because it’s a simple doubling of 1080p in each axis, whereas other 4K standards have different aspect ratios (some very different).
