I did 4 subjects at A2, and I would say that unless you're aiming for Oxbridge then it probably isn't worth it. It wasn't of any benefit to me to do 4 in the end, and I just gave myself more work. I did four because I got AACC at AS and both my Cs were messed up (damn you OCR) but my college was late on the ball in investigating it and nothing was done so I took both just to be safe.
If you thoroughly enjoy your subjects and will have the drive, willing to spend more hours in college (and out doing the additional work) compared to your friends, then go for it, but, as I said, I saw no benefit in doing 4.
Of course this is only my experience with it so I'd hear from a few more people before you make a decision.