Just in case anyone needs it, the (usually current) list of icon classes / font characters included in SS4 for use in menu classes, etc. can be found here: https://silverstripe-fonts.dorset-digital.net/
You can use the icon classes in your own ModelAdmin by specifying the class (eg.):
private static $menu_icon_class = 'font-icon-globe';
The character mapping section at the bottom shows which characters are mapped to the various icons when using the SilverStripe font.