Can a 16-year old work less than 4 hours a day?

I'm 16 and I was wondering if I could work under four hours a day? Do I have to specifically work the 20 hours per school week, or the 6 days per school week? Would anyone even hire me if I couldn't work on Wednesdays and Thursdays and every other Friday to Sunday because my dad has me on those days and he doesn't want me working on his days?