Moving average Google Sheets

I'm looking for a way to calculate the average turnover of the past 4 weeks.

The formula I used to determine to current year/weeknumber is:

=year(now())&"_"&0&(WEEKNUM(now();21))

The current year/week is 2021_05.

So I'm looking for a way to adding up the values of:

2021_01 + 2021_02 + 2021_03 + 2021_04

The number I'm looking for is the sum of these values (33.344 + 30.360 + 27.192 + 30.000) divided by 4.

Also the (moving) average should be excluded from zeros and empty cells.

Is there a way to calculate the average of the last 4 weeks in Google Sheets?