var urls_to_load = ["http://url.com","http://url2.com","http://url3.com","http://url4.com"];
var i = 0;
function loadIframeAndcheckIfReady(){
    var current_url = urls_to_load[i];
    alert(i);
    frame = document.createElement('iframe');
    document.body.appendChild(frame);
    frame.setAttribute('src',current_url);
    var inter = window.setInterval(function() {
        if (frame.contentWindow.document.readyState === "complete") {
          window.clearInterval(inter);
          i++; //Now we have one url more...
          if(i < urls_to_load.length)loadIframeAndcheckIfReady(); //recursively call the function until i it's iqual to urls_to_load.length
        }
    }, 100);
}
loadIframeAndcheckIfReady(); //Start process