If you've ever got 4 stars on AB you must be extra special The system only goes up to 3 ;-)
Whenever you answer a question, the questioner can click on 'Rate this answer' and choose whether to give it 1, 2 or 3 stars, depending upon how useful he/she finds it. There are no rules but, generally, 1 star means 'Why did you bother posting?', 2 stars means 'Thanks but you've not fully answered my question' and 3 stars means 'Wonderful. Thanks!
Alongside this answer you should see the words 'Rate this answer', so you know what to do ;-)
Chris