Pointless.
No University requires more than 3 A levels and you wont get any 'Ooo clever' brownie points for doing 4. AAA grades will always look better than BBBB
Too many people who attempt four A levels will not get the three grades they require - and every year thousands lose their University place as a direct consequence of being over ambitious. Whatever pressure or encouragement you are getting from parents and/or school to do four, try to resist this!