My son has been in speech therapy since he was 2 and, although he has never stuttered, his friends have. I was told this is a normal "phase" for some 3 and 4 year olds to go through. But, if you are concerned, you should get him tested at a private speech therapy place or talk to your pediatrician about your other options through the public school system.
Not starting speech therapy until they are 4 is absolutely untrue. Kids can start speech therapy when they are born if you want to. My son has been in speech therapy since he turned 2. There is a state program called "Birth to Three" that kids under 3 can go through. You should ask your pediatrician about it. Once you are 3, you can start speech therapy through your public preschool. Go get your child screened for preschool through the public school system to see if you qualify. In addition, there are always private agencies that see your children. They are in the phonebook.
If you are at all concerned about any area of development for your child, discuss it with your pediatrician. If you still aren't happy, there are many private therapy places around that can test your child to see if they are behind at all developmentally.