Although they generally only make offers on 3 subjects at A2, quite a few universities do want you to have at least a pass in another subject at AS. It won't hinder your chances too much if you don't do so well in it; in fact you'd be far more hindered if you didn't have one at all. When you apply to university you don't always even have to declare your ASs (depending on when you certificate them), all you have to do is provide the universities with your predicted A2s, so it's not so much of an issue if you didn't do so well, dropped it, but didn't necessarily want to mention your grade.