As mentioned earlier by another poster, a 4th bomber can spawn when you are burning down the walker if your group barely made the timer for killing the 3rd bomber and took some time to move to the walker to start the burn phase. This is due to the script thinking that you have failed to kill the 3rd bomber.
2 suggested ways to handle it when it occurs are to either have a tank taunt the 4th bomber and keep him occupied until he explodes on his own, or just have either a sage/sorc or merc/commando CC the bomber and let him blow himself up when his timer is up.
In the spirit of this developer fail game people post how to do work arounds for buggy raids. Kinda sad that this just goes on and on.