There is a report function for this. You also have no actual proof that it is an alt anyway.
Need admin 4pc help!

https://www.chess.com/variants/4-player-chess/game/33030373/31/1 he is most def not a alt...
https://www.chess.com/variants/4-player-chess/game/33030373/31/1 he is most def not a alt...
[Edit: screenshots removed]
Admin 4pc pls doing something with this alt account!