Please provide the abap code for this requirement

Question

note : if the below mentioned user exit is not suitable please find the suitable user exit and provide the code for this requirement.
•     The User-exit MEFLD004 is only to be used at PO level (ME21N/ME22N).
•     When PO is create or changed (ekko-ebeln) user exit MEFLD004 is triggered which check for the business requirement of the PO check for PO doc types (ekko-bsart) :z4,z6,z11,z12 from the table ekko and Account Assignment Category (knttp) is either N or K then stock process follows.  
•     When an PO is cancelled, all entries will have to be reversed
•     The Buffer table for the PO will have the following fields:
Purchase order Number                     EKKO-EBELN
Line item number of the PO               EKPO-EBELP
Vendor Number                                  EKKO-LIFNR
PO Quantity                                        EKPO-MENGE
PO Nett Price                                     EKPO-NETPR
Base Unit of Measure                  EKPO-LMEIN
Account Assignment Category          EKPO-KNTTP
PO Doc type                       EKPO-BSART     
Plant                            EKPO-WERKS     
Purchase Org                                  EKKO-EKORG
Purchase Group                    EKKO-EKGRP     
Company code                                EKPO-WAERS
Item category                     EKPO-PSTYP
PO Doc Date                      EKKO-BEDAT
•     No duplications are allowed for any created, changed or cancelled purchase orders. If such a case occurs, the record will be updated with the latest update time stamp. This custom table will be keyed by purchase order number.

Answer 1

Cross-post: http://forum.java.sun.com/thread.jspa?threadID=763485