Formula for a Moving YTD Total

Hi, can anyone help me with a formula?

Looking at the attachment, what I'm trying to do is use the drop down options in cells A2 and A4 to provide moving calculation points for both the current period date and the YTD totals. I've figured out how to do the current period but the YTD is eluding me. I'd like to only calculate the YTD value up to the cell value in A4 i.e if Period 3 is selected it counts periods 1,2 and 3 and if period 6 is selected it counts 1-6 etc etc. The data is pulled from the respective columns on the GRP sheet.