Can a dog reach sexual maturity at 4 months old?

My family got a male puppy and we already had two female dogs. We wanted to arrange to get them all spayed and neutered before he got to be 6 months old because that is when dogs usually reach sexual maturity. He is now four months old and we arranged for them to have their operations but one of the females went into heat. This morning i heard one of my dogs crying and i went into the living room and the male and the female were stuck together. Is is possible that the male could have reached sexual maturity already?