Don't even need to fight them simply enter the arena and walk out. The game will be scaled to four player until you save & quit.
Interesting. So that works for the rest of the game too? I go to fight one of them, then it puts me in 4 player mode?
If you trying to spawn Verm without four players and have the pirate DLC. I suggest visiting either Hyperius or Gees arena. As soon as enter their respective arenas the game will scale to four players. This will last until you save & quit.
This makes spawning Verm a lot easier, however it will make Verm and his varkid family much much tougher.