C.W.
HI A.,
I would suggest recognizing that it is a game your husband is playing and do not join in. YOur son is 4 old enough to know some things. So when you enter a game with him (the 4 year old) say " I do not play with people who don't follow the rules." Or I only play with people who play fair." so if you do not play fair I will simply get up and leave. Then if he cannot correct a 'cheat' or 'mistake' simply get up and leave reminding him of the deal.
You of course must allow he is only 4 and somethings don't appear to a 4 year old as cheating (you know when he is operating under a deceitful manner)
One more thing you may be helped with a book called "Thriving despite a difficult marriage." by Mike Misja
Good luck
C. W
LiveTotalWellness.com/Arizona