As Scott says, either PROC MEANS or PROC SUMMARY can summarize the data by any combination of CLASS variables.
However, I'm confused by the data. I don't understand how Jan 10, Feb 10 and March 10 are all week number 40. Unless the WEEK variable doesn't have any relevance to the actual DATE variable.
I may have misinterpreted your starting data and your desired results, but it seems to me as though WEEK is irrelevant in this context and the desired report is actually a summary based on CLIENT and SKU.