Hi,
I don't think you could make it work in the SQL statement.
What you could do is:
Add field PSTTR to the structure of GT_OUT and don't append the line if it does not match your condicition.
Something like this:
SELECT mara~mtart mara~matkl plaf~matnr plaf~plwrk plaf~verid plaf~psttr
INTO (gt_out-mtart, gt_out-matkl, gt_out-matnr, gt_out-plwrk, gt_out-verid, gt_out-psttr)
FROM plaf INNER JOIN mara ON ( plaf~matnr = mara~matnr )
WHERE plaf~matnr IN s_matnr
AND plaf~plwrk IN s_werks
AND mara~mtart IN s_mtart .
IF gt_out-psttr(4) IN s_mjahr.
APPEND gt_out.
ENDIF.
ENDSELECT.