The over-payment would be entered on the applicable line of Form 1040 and, assuming the employee did not owe any other Federal taxes, would be refunded to the employee. The employers who each paid $4,340 will not get a refund, since they are not aware that the employee overpaid in aggregate for the year. The government keeps the $818.40 overage.
Extra payments on Form 1040 would be recorded, and if the employee owed no other federal taxes, it would be returned to them. Employers who each paid $4,340 will not receive a refund since they are unaware that the employee overpaid for the year in total. The government keeps the $818.40 surplus.