I'm not a speech pathologist, just read what I could about it & have my own hypothesis - I think that it could be an inherited tendency, possible mix of DNA. This can be triggered by an incident with dominant male in the child's (4 year old's) life, possible a father, older brother, grandfather, uncle. The incident being an angry outburst (or out bursts) by the dominant male aimed at the 4 year old. The stammer is trigger as a psychosomatic response to illicit sympathy / empathy from the dominant male. The child is trying to affirm that they are not a threat to the older male's dominance. The stammer then gets locked in. So be gentle (but also firm), always maintain cool head. Being a parent is a long hard job, but I'm jealous of you as you're going to be making the best memories of your life. Sorry, I'm just an old guy rambling.