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.
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?