You said each row contains a total of sales data for the week, and
resides in column B. Assuming your first entry starts on row 1, to get
a 4-week rolling average you could simply use this formula in C4:
=SUM(B1:B4)/4
When you copy and paste that to the next row, the cell references will
shift, and consider only those 4 weeks of data.

Does that do what you need it to do?