I've seen loads of people with this issue but haven't found a fix yet. I can upscale the UI through the editor in-game which makes the game at least playable, but whenever I use my 4k monitor all the menus, names, and my mouse are tiny to the point where I can barely read the text.

If anyone's found a solution to this it'd be reeeeaally appreciated.

I can't help with the menus etc but you can try this to sort your mouse out: