<input id=input type="text">
<button id="b" hidden>GO!</button>

input.onkeyup= function(e){
  var len= this.value.replace(/\s+/g, '').length;

  if(len>16) return this.value= this.fix;
  if(len%4 === 0) {
    if(len === 16) {
      b.hidden= 0;
      this.fix= this.value;
      return;
    }
    this.value +=' ';
  }
}