No no no no no no no nonononononooooo!
It kills so much time! Honestly you're better of focusing on 3 subjects, doing maybe some voluntary work or extended projects and stuff like that, this will impress universities more!
Plus doing 4 subjects means more chance of getting worse grades at the end of it. I run out of time and i could've had A*s but got Bs because it wasn't physically possible to do everything and extra work. i mean maybe I have different circumstances.. which subjects are you doing? That plays a big role also.