So... no luck finding a 4k UI after scouring the forum, google, and eqinterface.com.
However, I found a solution, and wanted to update this thread in case anyone in the future wants to try this approach.
The solution is WinEQ
[direct download link]
Now unfortunately, you can't use the regular version to fix this. You will need to get a subscription to enable Fullscreen emulation. You can try a trial of it for 7 days to make sure it works. The pricing isn't bad ($15 for 90 days, $50 for 365 days, or $75 for 730 days). My rationale is that I'm able to play P99 for free in the first place, and the longest subscription is only $3.12 a month.
Hope this maybe helps someone in the future.