Lost Customer

Please try an expression like this:

Lost Customers =
VAR __3mosago =
    EDATE ( MIN ( 'Date'[Date] ), -3 )
VAR __4mosago =
    EDATE ( MIN ( 'Date'[Date] ), -4 )
RETURN
    COUNTROWS (
        FILTER (
            ALLSELECTED ( REVENUE[customer_id] ),
            AND (
                ISBLANK (
                    CALCULATE ( COUNTROWS ( Revenue ), ALL ( 'Date' ), 'Date'[Date] >= __3mosago )
                ),
                NOT (
                    ISBLANK (
                        CALCULATE ( COUNTROWS ( Revenue ), ALL ( 'Date' ), 'Date'[Date] >= __4mosago )
                    )
                )
            )
        )
    )

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat