Do try and do as well as you can in it, as it may well be beneficial for you UCAS application anyway. It isn't the end of the world if you don't do well in the 4th AS subject, though. Also, you may surprise yourself and do really well and want to carry it on. I know quite a few people who that happened to.
I did History, English Lit, Philosophy and Psychology last year, and knew I was going to drop Psychology from November onwards. I got a B in Psychology as I put the same amount of time in as my other subjects, and got As in everything else. I do feel like this was useful, as I got lower offers than other people applying with similar predicted grades to the same universities as me in a couple of cases.