Cheating is a predictable consequence of introducing young children to games and sports at a time when they can understand only the reward of winning but not the strictures against breaking rules of being dishonest.
As your young and inexperienced player sees himself outclassed in many situations, he is likely to change the rules in his favor or to cheat outright in order to win. His self-centered view of life makes it hard to put things in perspective, and winning becomes the most important thing at that moment. By the time he is six, he may have developed far enough intellectually and ethically to be intolerant of cheating in others, but he probably will not be consistent in applying the same standard to him.
Introduce your four year old to various kind of game and other competitive activities gently and in stages.Set aside some time regularly to play simple board games such as a carom board etc and put emphasis on the fun rather than on the competitive outcome. Simplify the rules to help him stay within them. Gently suggest a move now and then, so that he wins at least half the time in the beginning. If he loses too often, he may lose his self-confidence as well as the game.
Treat cheating as something that spoils the game rather than something your youngster should be deeply ashamed of. You might want to supervise new games with playmates, taking the role of good humored referee.
If your child or anyone else playing together needs to be corrected, you should do so with minimum fuss, so that the youngster is not humiliated before his peers.. If by six, your child cheats frequently, then you might examine what he has learned about competition.
Do not set excessively high standards. Think if you only praise him for winning something and treat losing as a form of personal failure. If so, your youngster might fall back on cheating as a solution to the situation you have put him in.