// ASPX (HTML) Part
<asp:CustomValidator ID="cvNotes" runat="server" Display="none" ClientValidationFunction="ValidateStringLength" ControlToValidate="txtSuggestion" ErrorMessage="Your suggestion is exceeding 4000 characters.">*</asp:CustomValidator>
// ASPX (Javascript) Part
<script language="javascript" type="text/javascript">
function ValidateStringLength(source, arguments)
{
var slen = arguments.Value.length;
// alert(arguments.Value + '\n' + slen);
if (slen >= 0 && slen <= 4000){
arguments.IsValid = true;
} else {
arguments.IsValid = false;
}
}
</script>