Go into your osu! skins folder, find the mania skin you want to change, go into the folder and find the skin.ini file.
Since you where playing 4k, scroll down until you find the "Keys:4" section, go to "ColumnWidth: 64,64,64,64" and change those 4 numbers to some smaller numbers like "ColumnWidth: 30,30,30,30".
Make sure you save the file and then verify the change ingame (use Ctr+Alt+Shift+S to quickly reload your skin). Your columns should be smaller but are most likely of centre.
To change the Columns positions (centre them), go back to the skin.ini file and under "Keys:4" change "ColumnStart: 300" so it is a larger number (higher = more to the right).
https://imgur.com/a/2xGruw9 (skin.ini bit)
Hope this helps.