In CR 8.5, create groups based on first 4 digits of Part_ID

I need to take an existing report and get sub-total of sales based on part type. i think best way is to break down the parts by aggregating the first 4 digits of the Part_ID. How do i do this?  TIA,,, Bill Here is the SQL of the report now:

SELECT
    RECEIVABLE_LINE."QTY", RECEIVABLE_LINE."AMOUNT",
    RECEIVABLE."INVOICE_ID", RECEIVABLE."INVOICE_DATE",
    CUST_ORDER_LINE."CUST_ORDER_ID", CUST_ORDER_LINE."PART_ID",
    PART."DESCRIPTION", PART."UNIT_PRICE"
FROM
    "SYSADM"."RECEIVABLE_LINE" RECEIVABLE_LINE,
    "SYSADM"."RECEIVABLE" RECEIVABLE,
    "SYSADM"."CUST_ORDER_LINE" CUST_ORDER_LINE,
    "SYSADM"."PART" PART
WHERE
    RECEIVABLE_LINE."INVOICE_ID" = RECEIVABLE."INVOICE_ID" AND
    RECEIVABLE_LINE."CUST_ORDER_ID" = CUST_ORDER_LINE."CUST_ORDER_ID" AND
    RECEIVABLE_LINE."CUST_ORDER_LINE_NO" = CUST_ORDER_LINE."LINE_NO" AND
    CUST_ORDER_LINE."PART_ID" = PART."ID" AND
    RECEIVABLE_LINE."AMOUNT" <> 0 AND
    CUST_ORDER_LINE."PART_ID" NOT LIKE 'NULL' AND
    RECEIVABLE."INVOICE_DATE" >= {ts '2011-03-01 00:00:00.00'} AND
    RECEIVABLE."INVOICE_DATE" < {ts '2011-04-20 00:00:00.00'}
ORDER BY
    CUST_ORDER_LINE."PART_ID" ASC