I also newly have invested in a 4k screen (size 32). The game looks fantastic in 4k.
Some thoughts regarding the UI in 4K:
1) I personally like to have the UI in 100% (except the mouse, see point 2). The UI becomes either unclear (not pixel perfect) or too big (fills up too much of the screen) if you turn it up in scale over 100%. The problem with the UI in 100% scale is that it is placed on the left side of the screen. It should be possible to put the UI in the centre. Or even better, it should be possible to place the different UI elements around the screen to your personal likings (but I understand that this is a lot of programming)
2) Like said in the OP the mouse cursor gets lost very often in the hectic game play because of the small size and that the cursor color is dark. It should be possible to adjust the size only on the mouse cursor (without scaling the rest of the UI).
3) My last wish (not directly related to this post) is I would like to be able to zoom out even further (than possible today) before it changes to the overview map. At least one zoom "level" more than now. I feel I loose a lot of the potensial from a big 4k screen with the limit today.