user White Tie is the "official" theme developer & has put a pre-version of the default theme, which will give you an idea of the potential, which hopefully should be fully realised pretty soon. Sorry nobody can be more precise than that, so right now you have a choice of going with WT`s theme or any of the other 200%/hi DPI themes offered or (sadly) waiting till reaper gets full Hi DPI support.
Main reason behind this I think is that they are two guys plus W.T. working part time & there are other priorities that are being addressed first.
Bear in mind I am just another user & this is just my take on it. Me? I am glad I have two regular monitors still & didnt jump right into 4k. More by luck than judgement!