I think your way of looking at it might be confusing you.
By multiplying the 4 hours by 2.5 you get 10, but not 10 hours.
4 hours is 4 hours.
If their standard day is 7.5 and they work 4 hours on a public holiday, there would be a remainder of 3.5 hours to be paid at base rate.
On a payslip this would likely look like this.
Your payroll suite might be able to automate these calculations.
The minimum shift duration depends on the award, I would recommend reviewing each award before implementing a minimum shift duration.
Below are some extracts from the fairworks website:
Public holidays - 4 hours for full-time and part-time employees
Public holidays - 2 hours for casual employees.
Each time a part-time or casual employee works, they have to be given at least 3 hours of work in a row.
I hope that all makes sense.
Edited at 29 May 2018 04:51 AM GMT