Is the employee, who is originally tasked with managing employee affairs, entitled to claim an additional salary for his work in the Financial Affairs Department, knowing that he continued in this additional work for 4 years?

If the claim is merely for mutual agreement and reconciliation, then the matter is broad. However, if the intent is to establish entitlement and prove a right, then that depends on the contract or agreement.

We understand that the employee performed additional work (managing financial affairs) without compensation, and that the problem arose after a long period. The employee could have refused or stipulated an additional salary, but he did not.

Therefore, the employee is not entitled to an additional salary unless he informed the employer beforehand that he was not working gratuitously, and the employer agreed to that; in which case, he would be entitled to a fair wage (Ajr al-Mithl) as long as they did not agree on a specific wage.