This sounds exactly like my son. When he started preschool the teachers had a difficult time understanding him. Finally, a month before he turned four, we had his speech evaluated by a professional.. and his articulation (understandability, pronunciation) was well-below average.
So.. we started private speech therapy. And guess what, 10 months later, this past Monday, he just graduated!
They re-evaluated his speech and he is now above-average. I feel so good about deciding to pursue this and how successful it was for him.
Now he can go into Kindergarten with confidence that the teacher, and most importantly his peers, will understand him.
I would definitely recommend speech therapy, and this is a good year to work on it. There is also free speech therapy through your state's Early Intervention program too. Good luck to you both!