I don't believe that a 4200 mah would fit.. even if you got it to fit you'd have too much weight and would probably stress the motor and esc... it would also be nose heavy as hell.

As to a 4s, more voltage will not increase flight times, they increase power, mah increases flight times. noob is right though, make sure you have a 4s capable esc and motor, you'd pretty much need a 45amp 4S capable esc and a more powerful, lower kv motor.

In a battery system think of voltage as the pump and the mah as the reservoir, the pump can push the water (current) more powerfully but does not affect how much is in the reservor (mah).