Tab is not working after textbox's textchanged event fired?

<script type="text/javascript">

  function KeyDown(e)

 {

    var e = e || event;

    var currKey = e.keyCode || e.which || e.charCode;

   if (currKey == 9)

   {

   document.getElementById("<%=Txtbox4.ClientID %>").focus();

   }

}

script>

<asp:TextBox ID="Txtbox1" runat="server"

  onkeydown="KeyDown();" >asp:TextBox>

<asp:TextBox ID="Txtbox2" runat="server">asp:TextBox>

<asp:TextBox ID="Txtbox4" runat="server">asp:TextBox>