That is, the user is prompted for fiscal week, and BEx will return sales between the first week of the period and the week entered by the user.
If this is your requirement you can try to restrict your SALES KF with below two variables.
0SALES KF
FISCWEEK(This is User entry based Variable)-4-FISCWEEK(This is User entry based Variable)
FISCPER(This should be derived from FISCWEEK(User entry Variable)
By doing above, you are restricting your KF with your FISCPER strictly and then you are applying an -4 offset with a range variable