It depends whether your amp is bridgeable or not. If it is, then you can certainly do it. The vast majority of 4 channel amps in the market are bridgeable. You should also be careful to bridge the amp according to the manufacturer's instructions, and also be aware not going below the minimum speaker impedance that the manufacturer recommends for the particular amp especially in a bridged configuration (this will be twice the minimum load reccomended in 4 channel configuration).