So, we had a bunch of these locks in the tables in this room, but no one knew the combination.
This method has definitely been documented elsewhere, but it required way too much googling for me to find, so I'm making my own guide for this method here.
Edit: I've tried this on some newer Defcon CLs, and it definitely did not work on them, so this is probably limited to older versions.
Step 1: Make Yourself a Shim
So, you want to get yourself a soda can, and cut a strip of the aluminum.
You want it to be about as wide as the plastic bit inside a usb connector is thick, and you want it to be as long as possible.
Step 2: Now Poke at the Lock
On the end of the lock, there's a piece that you can turn with a screwdriver while the lock is open to change the combination.
You want to take your shim, and stick it into the gap between that and the lock housing, on the side of the lock that has the button to unlock it.
Step 3: Some Theory
So, the way these locks (called a sesame lock) work, is that each dial has a notch in it. when you try and push the button to unlock it, a bar moves down, and tries to go into the notches in all four dials. If all 4 dials are all at the right number, then those notches are all in a nice straight line, right where the bar wants them to be, letting the lock open.
So, all we have to do is figure out a way to get all 4 notches lined up, then we can just rotate the wheels together, one digit at a time, until it opens. For this particular lock, however, people already figured out how far it needs to be turned once the notches are lined up, meaning that we just need to do that.
Step 4: Now Get Decoding!
So, if you slowly stick your shim into the lock, you should feel it hit something after it goes in 1cm or so. That something is the first dial of the lock.
Now, you want to pull the shim out a mm or two, turn the first dial up one, and poke at it again.
Keep repeating this until you find a hole in the dial, then insert the shim more until you hit the second dial. Now, keep repeating this process until you've put your shim through all 4 dials.
Step 5: Time for Math!
When you're done doing that, remove your shim from the lock, and subtract 3 from each of the four dials.
Step 6: Open!
And boom, it's open!
and if it doesn't open, try subtracting 1 from each digit. I had to do this the first time I tried this, maybe I'd just stuck the shim in crooked or something.