Well, it’ll be a while before a 4K BR disc is feasible, but a 4K MKV is very doable right now. But I’m sure regular SRT subs will be sufficient for that.
SRT files are never sufficient IMO, but I believe 1080p graphical SUP subs would work in that scenario. It’s a little player-dependent, though, so I’m hesitant to make a blanket statement that this will definitely work.
If people start reporting problems with 1080p subtitles with 4k video, I may accelerate this conversion process.