TinyMCE Fiddle

<script type="text/javascript">
tinyMCE.init({
    // General options
    mode : "textareas",
    theme : "advanced",
    plugins : "autolink,lists,spellchecker,pagebreak,style,layer,table,save,advhr,advimage,advlink,emotions,iespell,inlinepopups,insertdatetime,preview,media,searchreplace,print,contextmenu,paste,directionality,fullscreen,noneditable,visualchars,nonbreaking,xhtmlxtras,template,imagemanager,filemanager",
    
    // Theme options
    theme_advanced_buttons1 : "save,newdocument,|,bold,italic,underline,strikethrough,|,justifyleft,justifycenter,justifyright,justifyfull,|,styleselect,formatselect,fontselect,fontsizeselect",
    theme_advanced_buttons2 : "cut,copy,paste,pastetext,pasteword,|,search,replace,|,bullist,numlist,|,outdent,indent,blockquote,|,undo,redo,|,link,unlink,anchor,image,cleanup,help,code,|,insertdate,inserttime,preview,|,forecolor,backcolor",
    theme_advanced_buttons3 : "tablecontrols,|,hr,removeformat,visualaid,|,sub,sup,|,charmap,emotions,iespell,media,advhr,|,print,|,ltr,rtl,|,fullscreen",
    theme_advanced_buttons4 : "insertlayer,moveforward,movebackward,absolute,|,styleprops,spellchecker,|,cite,abbr,acronym,del,ins,attribs,|,visualchars,nonbreaking,template,blockquote,pagebreak,|,insertfile,insertimage",
    theme_advanced_toolbar_location : "top",
    theme_advanced_toolbar_align : "left",
    theme_advanced_statusbar_location : "bottom",
    theme_advanced_resizing : true,
    width: "100%",
    height: "400",
    setup: function(ed){
     
        ed.onKeyUp.add(function(ed,e){
        var editor_name =tinyMCE.activeEditor.editorId;
        var editors_to_be_matched = /first_editor|third_editor/;
        var matched = editor_name.match(editors_to_be_matched);
       
        if(!matched) return;
        alert("Do character count stuff here");
        // your code here
        
        })
    }
});
</script>

<form method="post" action="dump.php">
    
    <textarea name="first_editor"></textarea>
     <textarea name="second_editor"></textarea>
     <textarea name="third_editor"></textarea>
     <textarea name="fourth_editor"></textarea>
</form>