Play the Extra Stage : To play the 'Extra stage' you must do either one of these on your last stage :
4 Stars - Pass with all just greats.
5 Stars - Pass with all greats/just greats.
6 Stars - Pass with 100% of your groove meter.
7 Stars - Clear the song.
Flashing 7 Stars - Clear the song.
Unlike 3rd Style/7th Style/8th Style there are no special songs as your extra stage.