Hey, got a response from Riot support about this:
Greetings Who Counters,
My name is Zacharan and I'm with the tech team at Riot Games. I'm sorry to hear that you're having issues with the unit outlines being off. Unfortunately, this is an issue with the fact that League does not currently support resolutions above 1920x1080, so on 4k monitors there are visual issues. Reducing the resolution of your monitor, or playing in Windowed mode may solve the issue.
We are currently looking into supporting 4k monitors, so once League does support those resolutions the issue should resolve itself. Please let me know if you have any further questions!