I played 4K on a 27'' and got used to almost everything. The chat is a little small, but I think the map is arguably more useful in this resolution. The only problem was the mouse which was so small, it would get lost constantly making the game unplayable.
As of patch 7.24, the scaling for health bars was fixed for 4K. So, I figured I would put in a ticket inquiring about a fix for the cursor and see what's up. This is what the rioter Corndog said in response as of December 12th 2017. Hope this is a satisfying explanation.
"Unfortunately as you've said, League doesn't yet support 4k resolution. We hope to have it implemented in a later patch, but it's not something that can be discussed at this moment. As far as "why not just make the cursor bigger", it's a matter of how the UI scales as a whole. When the resolution is bigger than the game was designed for, it would be a matter of sizing everything about the game larger, not just the cursor to fit. You can change this in a way, by changing the game's video settings to play in Windowed mode at 1920 x 1080, which scales the UI to fit correctly. I understand that this makes the game smaller, but that would be the only compromise I would have to offer, aside from scaling your monitor down to 1920 x 1080, which downgrades your monitor's image quality."
