Thanks BabYagun! I was searching in the internet how to report just in case.
How To Report Cheaters

@Lt_Jilli, if you closed that game already, you need to open Game Archive:
It will show your games:
Click a game, then click a cheater name and then (i) icon.

Care to define or illustrate instances of cheating? There are no engines analyzing 4 player chess so I am unsure what "cheating" means.
@Healey76
Some types of cheating are defined within the rules tab for 4PC.
For FFA examples are telling others what to do/giving suggestions in chat, offering free points/pieces/etc.
Basically if you are using chat to benefit you during the game, then that is cheating. Chatting about moves and strategy AFTER a game is perfectly OK, however, and I would recommend it.
In teams it is almost impossible to cheat unless you hack or have it where your opponent's teammate is supporting you by trying to lose.
Hope this answers your question.
To report a cheater click on his/her name (during a game, after a game or in the Game Archive), then click (!) icon, select a checkbox and describe how they cheated.
(You can also use the old way: "Help - Report Abuse" menu. But in this case do not forget to mention game number in your message.)
New way: