Last4 Weeks DAX

If you are using a dimdate based from a sql table you can create a column in the dimension to calculate the weeks from the current date as an integer. We have a few week counters for the different ways to count weeks. The datediff in powerbi itself is limited in that it will not count ngeative date differences.

SELECT  dd.*
              , IIF(DATEPART(YEAR, GETDATE()) - 1 <= dd.year, 1, 0) AS 'Last2Years'
              , DATEDIFF(WEEK, dd.start_date_of_week, xx.StartDate) AS 'WeeksFromCurrentday'
              , DATEDIFF(WEEK, dd.iso_start_date_of_week, xxx.StartDate) AS 'WeeksFromCurrentdayISO'
              , DATEDIFF(WEEK, dd.bybox_start_date_of_week, xxxx.StartDate) AS 'WeeksFromCurrentdayByBox'
        FROM    dim_date_dsv AS dd
                OUTER APPLY ( SELECT    MAX(dsvx.start_date_of_week) AS 'StartDate'
                              FROM      dbo.dim_date_dsv AS dsvx
                              WHERE     dsvx.sql_date = CONVERT(DATE, GETDATE())
                            ) AS xx
                OUTER APPLY ( SELECT    MAX(dsvx.iso_start_date_of_week) AS 'StartDate'
                              FROM      dbo.dim_date_dsv AS dsvx
                              WHERE     dsvx.iso_year = YEAR(DATEADD(DAY,
                                                              ( 4
                                                              - DATEPART(WEEKDAY,
                                                              GETDATE()) ),
                                                              GETDATE()))
                                        AND dsvx.iso_week_of_year = DATEPART(iso_WEEK,
                                                              GETDATE()) - 1
                            ) AS xxx
                OUTER APPLY ( SELECT    MAX(dsvx.bybox_start_date_of_week) AS 'StartDate'
                              FROM      dbo.dim_date_dsv AS dsvx
                              WHERE     dsvx.sql_date = CONVERT(DATE,GETDATE())
                            ) AS xxxx;

Once you have this it's relatviely easy to set a new column in power bi to filter, we use 13 weeks but you can easily chnage to 4. 

The following column is on our dimdate table in powerbi to use in filters.

Last 13 Weeks ByBox = if(Dim_Date[WeeksFromCurrentdayByBox]<=13 && Dim_Date[WeeksFromCurrentdayByBox]>0,"Last 13 Weeks",if(Dim_Date[WeeksFromCurrentdayByBox]=0,"Current Week","> 13 Weeks"))