<form>
  <span id="billing_address_show"></span><br>
  <input type="hidden" id="billing_address_1">
  <input type="text" id="billing_address_2">
  <input type="text" id="billing_address_3">
  <input type="text" id="billing_address_4">
</form>

$(function () {
  $("#billing_address_2, #billing_address_3, #billing_address_4").on("input", function (e) {
    let text = $("#billing_address_2").val() 
      + $("#billing_address_3").val() 
      + $("#billing_address_4").val();
    
    $("#billing_address_show").html(text);
    $("#billing_address_1").val(text);
  })
})