True, but the PC outputs 4k to 3 screen (different image on each screen), effectively outputting a resolution of 12k
No, because it's missing the vertical resolution.
4k is 4096 × 2304, but 8k is not 8192 x 2304 (two 4096 screen next to each other), it's 8192 × 4608 (two next to each other, and on top of each other).
I don't know what 12k is, but I assume it's and extra row and column, but these people who make the "standards" like to fuck shit up.