Flap Sup:
I believe that the legislation on CA numbers is 1 per 50, or part thereof, passengers carried.
If only 150 seats are occupied, only 3 CA's are needed. If EZY were to carry 150 on every Airbus flight, they would have a load factor of 96%. The publicly stated load factor each year is some way below that, nearer to 80%. Therfore there are going to be very few flights that actually need the 4th CA. Should one be needed ever, I believe an airport standby CA will be utilised.