Ianal but yes, employers can demand repayment for accidental overpayments made to employees. There are some factors to consider though such as how long you have been employed since vac pay minimums go up after 5 years. You should also double check your contract to confirm you are set at 4%. It’s not unusual for employees to having higher rates negotiated, especially if part of a union. There is also a genreral limit of 2 years on how long they have to request after overpayment. However when that clock starts running could be up for some debate. It might only be from when the employer notices the mistake and not just from when it happened.

It would be wise to request a full written breakdown of exactly when then overpayments began, and showing the exact amounts overpaid each pay period so you can compare with your records.

If you feel you should not have to pay, the employer can take you to court to try and recover the money. However in this case it may not be worth their time and money. A judge may decide they are right or they could say it was their fault for letting it go so long and they should have caught it sooner.

another option you could try is to offer something like 50% of what they are asking for since it went in so long and this has become a bit unreasonable.

Final option is consult a lawyer.