My understanding is that the four cannon Spits were prone to the cannons jamming. The small tube running from the back of the exhaust stacks (MkVC) was an attempt to heat the firing and feed mechanism to prevent frost forming and jamming the mechanism.
Frost would form because the plane would be at rest in a warm temperature then ascend so quickly to intercept that frost formed.
They had mixed reports of success with this.
I believe that, when the four cannons did work, the firepower difference was notable.