The 4 weeks of leave that full timers get is 2 weeks twice. The downside is the leave includes what would have been days off. You're allocated leave using credit hours. The credit hours do not equate to an average days work - they are significantly less.
So, 4 weeks of leave are not 4 weeks worth of worked hours. It's about 3 weeks worth of worked hours and the shortfall for the 4th week you have to make up by working harder during the other days of the month - of take a pay cut.