No problem. And I pulled that out of my head. It should have read modulo operator.

Is there a way, we can do this using Forms error message or process error message please?

Hmm, I don't think so. This would be a javascript solution to the problem.

Here you go, assuming that you are using a "text under field option"

css

/* int-4 (could be named anything) is a class assigned to the input field you are targeting and the un-help class targets the text under lable */

.int-4 .un-help {
      color: red;
  display: none;
}

jquery

$(document).ready(function () {

// When the targeted field changes...

$(".int-4").change(function(){

// find the value of the field

 	var n = $(".int-4 input").val();
// make sure it's not empty
 if (n > 0) {
// find the remainder of the value divided by 4
    var r = n % 4;

// if it isn't 0, then show the text under label that says
// something like " Must be increments of 4!" and removes the 
// input value
    if (r !== 0 ) {
      $('.int-4 .un-help').show();
      $(".int-4 input").val('');
    }

// if the remainder is 0, hide the warning and move along.
    else {
      $('.int-4 .un-help').hide();
    }
  }
});
});