Dude, keep all four. I was also predicted AAAA for my Chemistry, Biology, Maths and Psychology AS levels, but having been screwed over by life completely I ended up bombing them and getting a U in the dreaded Maths.
By doing both Maths and Further Mathematics it seems you have both the ability and the commitment these subjects require which universities will totally find appealing. If you can cope with he workload, do four.
However, if you change your mind, try to change it early into your A2 course so if you drop anything you can pick up an AS in another subject.
...and how to stay positive