You can call the police, but it seems that the amount is not enough to file a case. You can find a friend in the public security bureau or something and give him a call, or go to him and tell him about the seriousness of the problem. He should be concerned and return it to you. Or if that doesn't work, just ask him to sue him. He should be honest enough to give it to you after receiving the subpoena. But leave room for others in everything, and also for yourself. If you don’t know the details of your relationship, if you still have many good memories and he has invested a lot of affection and money in you, then you should teach yourself a lesson. To take a step back, people's emotions and mentality are not good now. Don't be impatient for four thousand yuan. No matter what behavior the other party does to hurt you, this should also be taken into consideration.