It could be a phase. I believe as kids go through language gains, they can sometimes have a short term stutter. Or it could be stress related? If it continues, you could look into speech therapy. My 5.5 year old has a stutter. His stutter did start around 4 (late 3s or early 4s). He does go to speech therapy at school. He qualified for early intervention so hes been seeing the therapist since he was 4.5. He was actually the youngest our pathologist had started treatment on. (They typically like to wait to see if it goes away on it's own because once attention is drawn to it, the child may become more self conscious about it leading to a more severe stutter.) One thing I do at home is try to s l o w - d o w n especially when talking to him. Try to focus on taking breathing breaks when talking. Always allow them to finish their thought without interrupting. Dont talk over them or try to finish their sentence. Give them dedicated time to talk to you. We give our son time to talk to us about his day at bedtime without his sister around. This gives him time to talk without being rushed or without distractions and without his sister or anyone talking over him.