Use DAX to Last Average Four Weeks of Data

Hello, I'm trying to Google, YouTube, and otherwise stumble my way through learning DAX. What I'm having trouble with right now is this:

I've got a spreadsheet with the date, followed by item numbers. There are quantities for each item. What I'm trying to do is use DAX to take the quantities for the last four weeks and return the average. The date column is populated through the end of the year. There are no item quantities for future dates. The goal is to have the script start at the date where column for item "A" is not null, and average the quantities for the last four dates.

Any advice/assistance is appreciated.