Easy way to beat chapter 4
The last level is Deimos. Play to this point on casual, then switch to insane for the last level as mentioned on other threads.
Get 3 mates to join you, get everyone to head to the gun turret through the teleporter.
All 4 players need to switch to jackhammers and surround the telepoter while holding Right Trigger. Everytime they come thrgh the teleporter they meet instant death, even with full armour, as you bounce them between yourselves until they die.
The only risk is the redeemer can be fired into the room from outside. But this only happened twice to us and only killed 2 of us.
If you have not got 3 people to play you could try this same trick with just one other friend. I believe it would still work aslong as you get your positions right on the otherside of the teleporter.