If the kid is talking like that at four ,he's hearing it at home. Hard for me to have sympathy for Mom in a case like that.
We had a neighbor family where it was Grandma raising threee of her grandchildren. Sad story. Mom (Gma's daughter) killed in an auto wreck and Dad in prison. It was either her, or foster care. The older child (12 boy) and the younger one (girl 4) were difficult children. The girl had an unbelievably filthy mouth, and I'm certainly no stranger to profanity. I grew up surrounded by it and have been known to have a few colorful words fly out of my mouth on occasion.
Anyway, I remember being in the kitchen and a bunch of neighborhood kids playing in the front of my house. I hear this little girl voice just spewing all kinds of nasty language. I remember freezing. I go out and calmly explain that if she wants to come play at my house she'll have to stop with using such language. And, she did stop. I spoke to Gma about it and she explained that they didn't condone such language and when she did that at home she was punished for it.
It was some time later that Son and Daughter went up to their house to play video games. Daughter calls me from their phone to ask me a question. Daughter begins to talk and suddently I hear an adult SCREAMING in the background. It was blood-curdling! I was shocked and recongnized the voice belonging to Gma. She was screaming at her grandkids and it answered the question of where the four year old learned such words. I mean, my phone was vibrating with her screaming MFer and all the other words in that catagory.
I remember when she stopped, Daughter was silent. I finally asked "Are you okay?". A small voice said, "Yes", and then I instructed her to "get out of there, NOW!". She did and later explained that the Gma "wigged out". I asked if it happened before, she said "Yes, but never that bad". So, after that neither of my kids were allowed up there after that.
I also found out later that Gma's other adult children (no one was employed at that house), would teach the little girl all kinds of profane phrases because they thought it was funny to hear her say them.
They had all kinds of difficult child issues with that girl. Broken big screen TVs and sneaking out of the house while everyone else (nine people lived in that house) slept. I once found her in my back yard playing in a pile of sand was to be used in a house addition we were about to start. Thing is it was 6am on a Sunday morning. husband and I were very understanding and took her back home. Unfortunately, our patience ran out with them real quick-like. One of the last times I saw them was as they drove by and I was working in the front yard. She stuck her head out the window (of course, no seatbelt) and screamed, "FU 'B', kiss my 'A'". Nice, eh?
They lived on our street for two years. I won't go into all of it, but I had to call the police twice on the older difficult child. He broke Sons bike and then threatened me and called me all kinds of horrible names (whatever). Our neighborhood was elated when they moved out!
Anyway....I would suggest if you want to kill somebody, I would start with one of the adults. They are most likely the root of the problem. At that age, kids don't learn that stuff in a vacuum.
(by the way, that last statement was tongue in cheek and not to be taken seriously)