Re: How do you know if a line is too heavy for a rod?
A 6 is what I learned on, on sunfish, as a kiddo wading a lake shore. I recommend it unless your son is not yet up to strength to wield it.
A 6 will easily throw a popper to panfish - this is important - you want to set up your son for success; limit difficulty, kill frustration.
A 6, IMHO, will take more abuse than a 4.