Cumulative sum for weekly average hours over a month

I have a column called "Weekly Working Hours" which totals the number of hours worked per week. The cell is filled in every Saturday.

In the next column I have "Average Weekly Working Hours per Month" which needs to calculate the average number of weekly hours every four weeks, filled in every Saturday.

Please see attached file. I am referring to columns J and K.

A simple explanation of the solution would help too.

Many thanks,

Alis