NO no no! Do not let him change the rules or cheat!! That's just teaching all the wrong things. I know it's tempting...but don't.
If you want him to win (which is important at that age) more often than not, I think it's okay for YOU to purposely "do bad"...but don't change the rules or let him cheat.
With my six year old, for example, if we're playing Monopoly and she's going for one set of properties, I won't buy them up to prevent her from getting it, for example. But if she lands on my property...yes, she has to pay. I don't play anything as competitively as I would if I were playing another adult, etc...but she does win on her own, now, at least half the time.
If he doesn't get mad when someone else wins...I don't think you have a problem. I also don't think his age matters...he's four, he is capable of understanding why rules are there. If he really doesn't quite get it, then start making up all your own wacky rules to win and then explain that "see, that isn't very fair, or fun. Everyone has to have the same rules, so we can all have fun."
Good luck...and congrats on playing games with him!!! :) That's the best!