AZ Salary laws on minimum hours worked per day, paid for full day?

I'm apparently “Salary” when working 40 or more hours. Anything below that I'm considered hourly. There was one day last week I worked 4 hours instead of 8, so I lost 4 hours of pay on my check despite being “Salary”. A few weeks ago I worked an extra 4 hours and was never paid for it. Is there information somewhere that specifies my rights on this issue?