Is there really no way to change the menu font size?

I've just attached a 4k monitor to a mac mini (with retina). The image is large and great, but I can't find a way to increase the size of the font in the menu bar, which now is tiny. Of course I could reduce the resolution, but then I lose the advantage of having a nice, big monitor. I realize that Apple isn't selling it's own 4k monitor yet, so this problem probably isn't of interest to them, but is seems incredible that something as simple as font size can't be changed.

FYI I have tried TinkerToy and OnyX, but they don't appear to support this.

Any ideas? Have I missed something?

thanks