A TinyMCE 4.* plugin to add support for Typekit fonts - place in a new folder at tinymce/plugins/typekit/ and replace kitId value. Don't forget to add "typekit" to your TinyMCE "plugins" config array.

plugin.min.js

tinymce.PluginManager.add('typekit', function(editor) {
    editor.on('init', function() {

        // Get the DOM document object for the IFRAME
        var doc = this.getDoc();

        // Create the script we will add to the header asynchronously
        var jscript = "(function(d) {\n\
            var config = {\n\
            kitId: 'xxxxxxx',\n\
            scriptTimeout: 3000\n\
            },\n\
            h=d.documentElement,t=setTimeout(function(){h.className=h.className.replace(/\bwf-loading\b/g,'')+' wf-inactive';},config.scriptTimeout),tk=d.createElement('script'),f=false,s=d.getElementsByTagName('script')[0],a;h.className+=' wf-loading';tk.src='//use.typekit.net/'+config.kitId+'.js';tk.async=true;tk.onload=tk.onreadystatechange=function(){a=this.readyState;if(f||a&&a!='complete'&&a!='loaded')return;f=true;clearTimeout(t);try{Typekit.load(config)}catch(e){}};s.parentNode.insertBefore(tk,s)\n\})(document);\n\
";

        // Create a script element and insert the TypeKit code into it
        var script = doc.createElement("script");
        script.type = "text/javascript";
        script.appendChild(doc.createTextNode(jscript));

        // Add the script to the header
        doc.getElementsByTagName('head')[0].appendChild(script);
    });
});