How to get material revision level

To get material revision level use the function module "REVISION_LEVEL_SELECT".

See the below sample code.

    CALL FUNCTION 'REVISION_LEVEL_SELECT'
      EXPORTING
        aeobj              = '4'
        datuv              = sy-datum
        matnr              = wa_po-matnr
      IMPORTING
        arevlv             = wa_out-revlm
      EXCEPTIONS
        date_not_found     = 1
        ecn_not_found      = 2
        ecn_no_revision    = 3
        input_incomplete   = 4
        input_inconsistent = 5
        revision_not_found = 6
        version_not_found  = 7
        OTHERS             = 8.
    IF sy-subrc <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
    ENDIF.

Object 4 means for material. You need to pass the date, revision level on date. If you don't pass date you will not get results.