0) I think you ment Four Horseman?
1) Why not make extra locations with bosses? We could have a "Security Complex", filled with former humans, captains. At the end you would fight some pumped up former human (a Security Complex Operative?), surrounded with former commandos. That Security Complex Operative would be low on health, but have the ability to "mark" where the player is. This would cause former commandos to rush into the level. You would have to kill him quickly otherwise you'll get overrun by those commandos.
2) It has been a secret, that scientists were able to pass the portal earlier, many months before the invasion. It was possible to capture smaller daemons. Pinkies were great, because they were tolerant to high radiation, thus they could be sent to mines to retrieve resources.
The only problem was to control them.
"Animal lab" was used to train and add cybernetic implants into pinkies. The death rate was very high, but not among pinkies but scientist, who thought they just took control of them.
Player starts on a quite regular "city" like level. There are some former humans and pinkies. Somewhere in the level there is a locked "Cyberpinkie". He has three times the health, twice the speed and one armour extra then a normal pinkie. But for "Cyberpinkie" to be unleashed, player has to kill more than 75% monsters on level.
3) Bad dreams asylum
A place in hell where all nightmares are born. A mixture of cavern and city like level. It has nothing but lost souls and arch-villes. The boss of the level is "Lord of Styx", a pumped up arch-ville that has the ability to summon monsters. Remember, that the monsters he spawns are only in your imagination. If you defeat them, all the monsters will vanquish. Still, it is surprising how those illusion can harm you