Formula needed for running average

Need some help with a formula:

I need to see the running average to the current period, so looking at my attached spreadsheet I have periods P01 (cell D3) through to P13 (cell P3). In my example let’s assume that the current period is P03, cell B1 (this will change every 4weeks, at which point the value P04 will be entered at cell B1 and new data will be entered at row 4 & 5 for the current period).

What I need is a formula in cell Q4 that will average the results to date for that row (so if my current period is P03, I need the value in cell Q4 to return the average of periods to date) e.g. (10+3+4)/3

I need the formula to be automated such that I enter to values for the current period, amend cell B1 to the current period & the formula in cell Q4 will return the average of the periods to date.

Thanks in advance for help.

Thanks that does exactly what I want. Understand the formula apart from the D4 prior to the index, can you explain please.