"4K" was pretty widely used in the cinema world well before it really started to become something that came into the consumer market. So for once I don't really think this is some hidden marketing agenda
Personally, I think it's close enough to 4000 for 4k (and 8k by extension) to be a perfectly acceptable short hand. I'll take having to say "4k" any day over something like "WQUXGA". Particularly since these goofy names don't give you a lot of other information that can be just as important (Color depth, chroma sub-sampling, color space, refresh rate, etc). If you want to be specific, it's much more clear to just write out the resolution, aspect, etc. If you don't need to know all those details, saying '4k' is probably good enough.