A small startup company wishes to know how many hours, per week, that its employees spend commuting to and from work. The number of hours for each employee are shown below. Construct a frequency table for grouped data using four classes.

13, 5, 4, 16, 13, 6, 16, 4, 23, 21, 7, 12, 23, 13, 5, 8, 8, 7, 13, 16