Hi, and welcome to PC...

Perhaps your dream of your son at this age represent an exceptionally good period in your life. Maybe your son is a detail that doesn't necessarily represent your "son", but instead of something of great, pleasureful meaning to you.
OR, it can mean just the opposite, depending on the nature of the dream.
Maybe his age of 4 represents something which happened at that time which is nagging your subconscious?

Maybe, try to analyze what may have taken place the year your son was the age of 4. Perhaps you can tap into something which happened that you have since forgotten?

Hope that helps some~

Shangrala