How do I launch datepicker from a textbox based on the drop down item chosen?

How do I launch datepicker from a textbox based on the drop down item chosen?

I am working in ASP.NET 4.0 and use IE8, Firefox and Chrome for testing.  Inside the web page I have a textbox and a drop down with 5 items.  If the 4th item is chosen I want the textbox to become a datepicker.
 
Here is my best attempt thus far.  Any help or advice is greatly appreciated.

$(document).ready(

function () {

$(
"#txtSearch" ).focus( function () {
if ($( "#ddlSearch" ).attr( "selectedindex" ) == 4) {
$( "#txtSearch" ).datepick( "enable" );
} else {
$(
"#txtSearch" ).datepick( "disable" )
}
});
});

<

link href="Styles/jquery.datepick.css" rel="stylesheet" type="text/css" />

<

table width="100%" align="center">
<tr height="32px">
<td>
<asp:TextBox ID="txtSearch" runat="server"></asp:TextBox>
</td>
<td>
<asp:DropDownList ID="ddlSearch" runat="server" AutoPostBack="False">
<asp:ListItem Selected="False" Value="1">option1</asp:ListItem>
<asp:ListItem Selected="False" Value="2">option2</asp:ListItem>
<asp:ListItem Selected="False" Value="3">option3</asp:ListItem>
<asp:ListItem Selected="True" Value="4">option4</asp:ListItem>
<asp:ListItem Selected="False" Value="6">option5</asp:ListItem>
</asp:DropDownList>
</td>
<td align="left" width="100%">
<asp:LinkButton ID="cmdSearch" runat="server" CssClass="BUTTONIMAGE">Search</asp:LinkButton>
</td>
</tr>
</table>