Need help with CASE statement in Function

<div class="Section1"><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Dear friends,</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">I am a new user to postgreSQL and really need help to solve my “stupid ?” problem.</span></font><p
class="MsoNormal"><fontface="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">I have created  function with 4 arguments like this :</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Courier New" size="2"><span style="font-size:10.0pt;
font-family:"Courier New"">CREATE OR REPLACE FUNCTION "public"."fHistoryCard" (begdate date, enddate date, ProductID
varchar,storeID varchar) RETURNS SETOF "public"."tbltrans" AS</span></font><p class="MsoNormal"><font face="Courier
New"size="2"><span style="font-size:10.0pt; 
font-family:"Courier New"">$body$</span></font><p class="MsoNormal"><font face="Courier New" size="2"><span
style="font-size:10.0pt;
font-family:"Courier New"">/* New function body */</span></font><p class="MsoNormal"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt; 
font-family:"Courier New""> </span></font><p class="MsoNormal"><font face="Courier New" size="2"><span
style="font-size:10.0pt;
font-family:"Courier New"">select * from tbltrans</span></font><p class="MsoNormal"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt; 
font-family:"Courier New"">where tbltrans."Date” between $1 and $2</span></font><p class="MsoNormal"><font
face="CourierNew" size="2"><span style="font-size:10.0pt; 
font-family:"Courier New"">and upper(tbltrans."ProductID")=upper($3)</span></font><p class="MsoNormal"><b><font
face="CourierNew" size="2"><span style="font-size: 
10.0pt;font-family:"Courier New";font-weight:bold">and tbltrans."StoreID"=$4</span></font></b><p
class="MsoNormal"><fontface="Courier New" size="2"><span style="font-size:10.0pt; 
font-family:"Courier New"">order by tbltrans."Tanggal";</span></font><p class="MsoNormal"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt; 
font-family:"Courier New"">$body$</span></font><p class="MsoNormal"><font face="Courier New" size="2"><span
style="font-size:10.0pt;
font-family:"Courier New"">LANGUAGE 'sql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;</span></font><p
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font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">---------------------</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">My Question is : How to make argument 4 optional ? When IS NULL the function will show all
transactionbetween date $1 and $2 and product ID=$3 </span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">But when $4 is not null then the fucntion will show all transaction between date $1 and $2 and
productID=$3 AND STOREID=$4.</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">I really appreciate any suggestions.</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Thanks a lot.</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Hengky</span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font></div>