Can a spiral fracture in the humerus near the elbow happen in a 4 month old when the arm is grabbed by an adult man to keep the child from falling?

Yes it could: Yes it could happen depend on how the adult hold the infant at the time of the fall.

Possible: It is possible but one must always worry about child abuse.

I wouldn't buy that: You don't grab an arm or elbow on a 4mo to stop a fall, you grab their body. Spiral fractures result from a twisting motion. While it might be possible in theory, the kid deserves a full body scan for other injuries and the family should be evaluated by social service for stressors. I would hospitalize such a kid for safety until a full evaluation was completed, using a court order if required.