Validation control for entering numbers

You Can use it by two ways:-

1. Javascript
just add this script on page's HTML side

<script type="text/javascript">
function checking()
{
  var check1 = document.getElementById("<%=yourtextboxid.ClientID %>").length;
  if(var<>4)
  {
        alert('Please Enter Only 4 digit no.');
        return false;
  }
   else
    {
   return true;
    }
}

//this function is stopping the albhabet values has to be entered
function CheckNemericValue()
        {      
        if((event.keyCode >= 48 && event.keyCode <= 57) )
        {
        event.keyCode = event.keyCode; 
        }
        else
        {
        alert("Only Numbers are allowed");
        event.keyCode = 0;
        return false
        }
        }
</script>

and on page load event

protected void Page_Load(object sender, EventArgs e)
{
//this will invoke your javascript function on keypress event of textbox
yourtextboxid.Attributes.Add("onkeypress", "return CheckNemericValue(event);");
}

and on Button just add this javascript function like this.

<asp:button id="Button1" runat="server" text="Add" onclientclick="return checking()" onclick="Button1_Click" xmlns:asp="#unknown" />

this will help you.

and the
2. By validation controls

use

<asp:regularexpressionvalidator id="RegularExpressionValidator1" runat="server" errormessage="Please Enter numeric Value Which contains 4 Digits Only" controltovalidate="yourtextboxid" setfocusonerror="True" validationexpression="^([0-9]{4})$" validationgroup="validation" xmlns:asp="#unknown">

You can also set text box's maximum length=4. so the user can't input more than 4 digit.//this is optional

tell me if for any error.

Mark it as Answer if it help you.