The default mediavps_2014 config will definitely have issues at 4k with being too small. There's a few alternate HUD configs around that are designed for 1080p, they may work better, but even they might be too small.
Having said that, there are reports of other issues with FSO running high resolutions, IIRC something about the offscreen reticle and/or the secondary lock reticle not behaving. And due to the current devs lacking monitors with a high res, it's hard to debug & fix the issues. Therefore I'd recommend crossing FSO off the 4k capable list (for now).