Our shortcode dropdown box in Tinymce is not working in WordPress 3.9?

I’ve been stuggeling with a similar issue. (button not appearing in mce editor toolbar). This pattern worked for me in WP 3.9 / tinymce 4.0:

tinymce.PluginManager.add( 'thing' , function( editor ){
    editor.addButton('thing', {
        type: 'listbox',
        text: 'My listbox',
        onselect: function(e) {
            // do things...
        },
        values: [
            {text: 'Menu item 1', value: 'Some text 1'},
            {text: 'Menu item 2', value: 'Some text 2'}
        ]
    });
});

Sad enough this is not backwards compatibele, so you will either break 3.8 support or you will need to implement some version sniffing.