This is probably obvious to many who played Halo: Reach more extensively than I did but I actually had some trouble figuring out how to use the fuel rod cannon on the Banshee in Halo 4.

The solution is ridiculously simple: Hit the ‘Y’ button to switch from the the Plasma Cannons to the Fuel Rod Cannon, similar to how one switches weapons when not in a vehicle.

That’s it. The reticle will change to indicate the current firing mode.