Hello,
Create a blank variable and apply input control on this blank variable by manual entries. using "Right()" formula, get the week numbers and use another variable with same "Right()" function to get the week number of the selected week. create a measure as shown in below to get previous 4 weeks values and plot it in graph.
In below example only difference is its not fixed to previous 4 weeks. Here it plots all the previous weeks unlike your requirement.
SAP BO WEBi. Trend of Present + Past 7 Weeks
Hope it helps.
Regards,
Nikhil Joy