solusconvert it defaults to 30 Hz refresh rate in the display settings
I have MATE on the computer that drives my TV, which is a 55" monitor using the same 4K resolution you're using. In my case the display settings offer 24, 25, and 30Hz as the refresh rates. Of course, I leave it on 30Hz and it never changes when I reboot after an upgrade session.
Obviously it isn't Solus that determines the maximum refresh rate; it has to come from the monitor according to its capabilities. (I suspect it's part of the plug-n-play feature.) The owner's manual for your monitor is probably online, in case you don't still have a printed copy. Does it say anything about a preferred refresh rate, or a maximum refresh rate in the manual?
Maybe there's a clue there as to why this is happening?