I owe $ 4000.00 to the deceased now person. It was a part of the separation agreement (merital debt division) that I should pay after the divorce is final. But the person, whom I was suppose to pay died. What should I do?

It is technically owed to his estate. So it is owed to his inheritor.

You need to find out who the executor of the estate was/is and arrange with that person to make payment. You can check with the probate court where he died to find out who the executor might be.

Also if this is a debt that you were making payments on for the interim you can put those payments into an account and hold them separate until all this is resolved. But as the others have said you should notify the relatives. If you don't know them then the place to start would be the funeral home. They could either tell you or allow a letter to be passed to them so contact can be made. Just be sure to explain the situation.