Sorry for the late response…
I wasn’t able to test the 4k output full-screen on a 4k screen directly out of VLC because the highest res screen we have to develop on is only 2K. That said, the actual videos are only occupying a small part of the screen – effectively making them HD or less in a 4k XP. We do run into some artifacting while running the XP at 2k, but mostly only when I layer video with, say, a slide show. Given all that, I’m really thinking it’s a processing power thing. They keep threatening to buy me a 4k screen to run the XP, but so far they haven’t been able to get a screen larger than HD.
Which makes me wonder how much CPU is being used to scale the whole XP down to HD – and whether that is also affecting performance…?