Yes, your son is likely gifted. Does he have other outlier abilities that can give you more clues about his level? Advanced legos perhaps?

And just from my experience in my childrens' pre-ks (with highly motivated parents), most 4 year-olds, no matter how well-taught, won't be able to read MTH books. My dd is in an HGT classroom this year in 1st, and some of the HGT kids are barely reading MTH.