time series

Hi, just wondering whether someone can confirm how to do this properly, given a tabulated set of values of a time series for 4 quarters in each year 2004-2008, I've been asked to apply a centred 4 MA smoother to the time series and then construct the seasonally adjusted time series from this. I think the centred 4 MA is just averaging each year over the 4 quarters, so doing this I now have 5 averaged values corressponding to the 5 years in question. Does anyone know how I now go about constructing the seasonally adjusted time series using these values? Thanks