hey what did you get in your GCSEs? I usually recommend against 4 a levels. Most unis only look at three of your grades and doing an extra a level will very rarely put you at an advantage to those who do three. I was originally thinking about taking further maths as a fourth subject but ended up choosing against it after doing more research. Unless you WANT to do a fourth just because you really want to and enjoy the subject i would advise against it. This way you can put more time and effort per subject giving you better results, which actually affect your application.