Just because it sounds like your 4 year old son in cussing like a sailor in the back of your car doesn't necessarily mean that is the case.
The other day on the way home from school I thought I heard Evan saying f*** then s*** over and over. Jake and I looked at each other like noooo it can't be our sweet little boy with such an awful potty mouth. Just as we were about to question Evan on where he learned such language it became very clear to us what he was trying to say. He was sounding out the word function. Why was he trying to say the word function? Not sure, but at that point I really didn't care, I was just glad that my little boy didn't have a mouth like a biker!