My ds (also 4) has been slightly hurt/minor scratches and cuts etc by a few toys. He's fallen off the trampoline too because he was walking on the outside of the net. We don't take the toys away, we just explain how his misuse or rough handling has impacted on him. It was just an accident, no toy is 100% safe. I wouldn't consider returning it, just tell him not to put his finger near the wheel in future.