They're more likely to say something like AAA etc. and sit fourth A level. However, remember unis will look at your predicted grades. Also, you don't want the fourth subject to bring your other grades down (A*AA is far better than AAAA for example). Also, if you think you won't do well (but it sounds like you will) then what's the point? It might look bad for future careers. I was in a similar situation to you, but I want to go on to do law and further maths is not of very much relevance so I dropped it. So I guess it depends on your degree, for example if you want to go on to do physics I would definitely keep further maths.