I know this is an old topic but I feel it needs to be resurrected. League is currently unplayable with a 4k monitor. It's also asking a lot of your customers to re scale our desktops just to play your game. This needs to be a higher priority.
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."
