Moving Average Ecluding Blank Cells

I have a column of data that contains various blank cells where no data was measured. In the adjacent column I want to take the moving average of the last 4 data points including the most recent entry. My problem is i do not know how to handle blank cells where there was no data. I need it to average the last four in the column where data acutally exists. I am ok with using helper cells if needed and I am not worried about the first four results at this time.

Example data

A..................B
15
50
25
20................55
Blank............55
30................31.25
35................27.5
blank............27.5
blank............27.5
15................25
10................22.5
15................18.75
40................20
blank.............20