This is way beyond anything I could come up with a home remedy for. At 4 years old he should, without a doubt, be talking. I'd suggest you take your son to an expert on child development and speech. There are so many areas to test and study, it could be from a trauma he may have experienced as a baby.
I'm sorry, I wish I could have been more help to you but when dealing with children it's always best to find experts in their field.