Act 4 is called "Helheim" and is made up of 5 sections: 4-1, 4-2, 4-3, 4-4, and 4-5.
This level differs from the others by having 2 bosses:
At the end of 4-3 you will fight a giant dog called Garm. This guy is fairly easy to beat. Target his various sections and shoot at them from a distance until each section has lost all armor/health. If you stay at a distance, the only attack he will use is a needle barrage that you can either roll away from or jump over.
At the end of section 4-5 you will fight your last boss, Hel. She is a push- over compared to the other bosses. You will meet her in a room where she will appear in the center. Attack her with melee for a bit, and then she will teleport around the room.
She has 3 main attacks:
She will spawn some minions to get in your way.
If you are close, she will do a punch-kick combo on you.
She will fire a purple laser beam at you, which will constantly drain your health.
Keep attacking her while she teleports around the room, and when she has lost so much health, she will disappear and you will need to move into the next room.
The second room is exactly the same as the first room. Keep attacking her, and when she gets down to so much health, she will disappear again.
Walk up the ramp, killing enemies on the way, and enter the cage arena at the top. Continue to attack her like you have in the first two rooms. She will die fairly quickly.