You need to pick a fourth A Level you really want to do. Not to discourage you, but I did four and it was hard and I would only recommend doing it if you really need to. I did Biology, Maths, Chemistry and Music (and music is a hell of a lot harder than people think). Having a fourth that was different only helped me because I was applying for veterinary and zoology, which require you to be able to separate home and work life and they analyse your personality quite closely for that. Having a separate fourth a level isn’t necessary (only a few people actually take that many) if you are able to provide evidence of other interests in your personal statement, like clubs etc. They mostly like to see a wide range of interests to make sure you wont sacrifice your wellbeing to bury yourself in work. So, if you are going to choose four, pick four you will enjoy and that won’t be too much work. A Levels should be enjoyable, not just a step to what’s next!