Actually, 60D of the Employment Act, does not state about replacement when a worker is required to work on public holiday, it only state about how he is to be paid. Note the words "...regardless that the period of work done on that day is less than the normal hours of work. " As such, he should get 2 days salary for working the 3 or 4 hours. So, in your case, if it is an accepted practice to get a replacement, you better give a replacement or pay two days salary for working on that day. You may consult the Labour Department also.
(3) (a) Notwithstanding subsections (1), (1A) and (1B), any employee may be required by his employer to work on any paid holiday to which he is entitled under the said subsections and in such even he shall, in addition to the holiday pay he is entitled to for that day:
(i) in the case of employee employed on a monthly, weekly, daily, hourly, or other similar rate of pay, be paid two days' wages at the ordinary rate of pay; or
(ii) in the case of an employee employed on piece rates, be paid twice the ordinary rate per piece:
regardless that the period of work done on that day is less than the normal hours of work.