4 years old seems awfully young to start filling your sons head with a bunch of psychobabble. Just let him be a kid and don't make a big deal about him stuttering. The worst thing you can do is put the idea in your son's head that he speaks wrong and needs to stop doing it. Stuttering really wouldn't be so bad if it weren't for all the self loathing that comes along with it. That self loathing comes from a lifetime of trying not to speak wrong and failing at it. Don't treat his stuttering like it's something that needs to be fixed. Treat it like you would left handedness. Nothing to be concerned about. He's still young enough that it could go away on it's own. But the more he thinks about it and views it as a problem, the less likely that becomes. Seriously just let him be a kid who thinks about normal kid things as much as possible.