Well, if he has never been left alone, then this did not happen. But, then the question is where did he get these words. That is not something that 4 year olds make up out of the blue. Is he at a preschool where he has a lot of playmates that might have been saying this?
It seems impossible that a child this young could know about this type of sex act. It only takes a few minutes without supervision for a child to be molested. Was the child and the 14 y o ever in the house at the same time? If the answer is yes believe the child NOT the perpetrator.