So, could i maybe suggest to you and the skinning community, that it would be great to see some blinds coming out also with a substyle to suit this?
First I don't know of any Skinner that has the ability to even see what a WindowBlind would look like on a 4K screen. Without being able to view a blind in progress on 4K with the screen resolution it has, making a different sub style for it would not even be possible. Saying that it goes way beyond that. Images used when making a WB can be made to different sizes. But these sizes are limited. You can't just make a graphic that for example is say 224x30 pixels in size and make it who knows how big so the buttons would show. Well, maybe you can but how does one check to see what it looks like on 4K.
I have a suggestion and i'm not trying to be a smart ass with it. How are the Min, max, etc. buttons showing using IE, Chrome, FF for example? I would think they would have the same problem. You said you have made skins in the past but no time now. I'm sure you could find at least some time to just mess with different size title-bar buttons to see what the final look is. I really don't think it's a graphic size problem but a resolution problem, but again not sure.
Bottom line here is a Skinner can't even try to see if a new sub could be made for 4K resolutions. you can as you are using 4K. Doubt very much if you will find people using computers that have the ability to view what they are doing on a 4K screen.
If you are able to find this person, maybe work something out to pay them for their services. Sounds like the main problem is text size and titlebar buttons. Start with that first to see what can be done, if anything.