Average Flow Rate Over an Interval

The rate at which water flows through a pipeline, in gallons per minute, is given by the function $Q(t) = 30 + 8 \sin\left(\displaystyle\frac{\pi t}{4}\right) + 5 \cos\left(\displaystyle\frac{\pi t}{2}\right)$, where $t$ is the time in hours after midnight. What is the average flow rate, in gallons per minute, on the time interval $0 \le t \le 4$?