4K is 2160p, exactly double 1080p in the vertical. In the horizontal, it’s 1920 for 1080p vs 3840 for 2160p (colloquially known as 4K). The industry naming convention is not consistent.
4k (a cinema format): 4096x2160
2k (a cinema format): 2048x1080
UHD (a broadcast format): 3840x2160
HD (a broadcast format): 1920x1080
UHD also includes 7680x4320 *HD also includes 1280x720