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Thank you for the additional information.
It sounds like this is what happened: You work for an employer that had economic problems. The employer decided that you and the other employees needed additional schooling that would cost $4000. The employer paid for the school You started the school. Two weeks later, the employer asked you to sign an agreement that if you left employment within a year, you would repay the $4000. You did not want to sign the agreement, but your employer said that the school was mandatory. (Presumably, you would have been fired if you didn't). Therefore, you signed the agreement and completed the school. You resigned from the company later, before a year had passed, because you were not getting paid the overtime you had gotten before.
This is a complex situation. It appears that the contract that requires you to pay back your former employer would not be enforceable because (1) you started the training, and then were told that you would have to pay back the cost of the school; and (2) you signed the agreement under duress. Those would both be defenses if your former employer tried to sue you to collect the costs of the school.
If the Company is trying to collect from you now, you should have an attorney in your area review the agreement you signed. There could be additional clauses and provisions in the agreement that might affect your situation.