Force 1/2 day vacation increments for full-time employees

All:

Currently, full-time employees are are those that put in 37.5 to 40 hours a week. They are allowed to take increments of 1/2 day or full day vacation time. The 1/2 day vacation equals either 3.75 or 4 hours. Is there a way for the system to disallow taking less than 3.75 hours half day vacation for full-time employees? In other words, force full-time employees to use full 1/2 day which is either 3.75 or 4 hours.

Thanks,

Anil