Need help with CASE statement in Function

Dear friends,

I am a new user to postgreSQL and really need help to solve my "stupid ?"
problem.

I have created function with 4 arguments like this :

CREATE OR REPLACE FUNCTION "public"."fHistoryCard" (begdate date, enddate
date, ProductID varchar, storeID varchar) RETURNS SETOF "public"."tbltrans"
AS

$body$

/* New function body */

select * from tbltrans

where tbltrans."Date" between $1 and $2

and upper(tbltrans."ProductID")=upper($3)

and tbltrans."StoreID"=$4

order by tbltrans."Tanggal";

$body$

LANGUAGE 'sql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;

---------------------

My Question is : How to make argument 4 optional ? When IS NULL the function
will show all transaction between date $1 and $2 and product ID=$3

But when $4 is not null then the fucntion will show all transaction between
date $1 and $2 and product ID=$3 AND STOREID=$4.

I really appreciate any suggestions.

Thanks a lot.

Hengky