Ultimate Doom: 4th Secret Level - "Fear"

The 4th secret level can be accessed from E4M2 - "Perfect Hatred."
Run passed the cyberdemon (if you can) until the door containing the swarm of cacodemons and the exit will open. A teleporter behind you will also be revealed. Enter the teleporter to telefrag the cyberdemon. The highered floor in front of you is a platform. Once you activate it, another platform will lower back in the lava pit area. Stand where the radiation suit is located. You should be able to see the teleporter to the left and a wooden corner to the right. That corner is the platform that lowers. Beyond there is the switch that leads to the 4th secret level, "Fear."

Cool. Thanks. I now take back what I said about them putting a cyberdemon in a box (several psts back). Thats cool. BTW, The only time I ever telefraged someone was on co-op. OOPS!

You can telefrag the Cyberdemon, btw. there's a teleport somewhere that takes you to the cyb and obliterates him. I don't remember where though.