10gb should still be fine for basic 4k, for maxed out 4k possibly not. And maxing out is kinda pointless unless you have something daft like a 3090 or 6900xt.
Not quite sure. Pretty convinced the 1440p guys on here were saying that it was near or close enough on some games. Would be better if some real application could wade into the VRAM actual vs reserved and measure what games are commanding then including it in these reviewers benchmarks.