Last 4 Week formula

Hi, I need to write a formula to fetch last 4 week data in webi report.I have Dim_Week_no which has whole year data.from 201701 to 201852 I have created a variable for Last4Week: =Substr([Dim_Week_no];1;4)+""+Week(RelativeDate(CurrentDate();-28)), my completed week is 201802, so for last4 week i need data from 201751-201802. Last 4week isn't working for me =Substr([Dim_Week_no];1;4)+""+Week(RelativeDate(CurrentDate();-28)) giving me 201751 and 201851 please let me know the solution