The Ray Gun mark 4 is a custom wonder weapon in this map. It is a Ray gun/shotgun hybrid, and is a good room clearer, and good for taking out the police boss.

Step 1: Turn on Power (if you can't do this, look at my Easter Egg guide. It is detailed there)

Step 2: Activate all the switches. The switches around the map require power to be on to be flipped, and there are 7 of them.
Switch 1: In the spawn room to the left of the M14
Switch 2: In the Gen 2 area, by the gate to the Main Street.
Switch 3: In the room between the Urn cleansing and the Cargo Area
Switch 4: In the Cargo area, behind the opened up Cargo crate.
Switch 5: On the main street, to the left of Dr. Carbines room.
Switch 6: Inside of the Speed Cola room.
Switch 7: In the bathrooms across from Mule Kick.

Step 3: Head over to Dr. Carbine's room. He will instruct you to bring him a syringe and a health pack.

Syringe: This syringe is hard to miss, but it is on a lonely sink in the bathrooms, right across from mule kick.

Health Pack: The health pack looks like a bunch of pill bottles, and is located on a box to the right of stamin-up in the market area with gen 4.

Step 4: Bring these items back to Dr. Carbine's room, and he will give you a Ray Gun mark 4. In Co-op, only one player can grab it.

Hope this guide helps you guys.