Bentley: What's with this industrial-strength voodoo gate? Mz. Ruby must really be trying to keep something out.
Sly: Or maybe... she's trying to keep something in!
― Bentley and Sly while entering the Beast's lair[src]
Sly entered a large swampy area, which happened to be the lair of a large swamp beast.
Sly discovered a gigantic voodoo gate that Mz. Ruby apparently built to keep something from entering or escaping. Sly climbed up and around the gate in order to steal the treasure key that was located in this area.
Making his way through the swamp, Sly discovered that the gate wasn't meant to keep anything out; rather, it was built to keep in a massive sea serpent, The Beast. When Sly approached its hiding spot, the creature awakened and chased Sly through a thicket, with Sly barely making it to the key platform before the Beast dived back into the swamp. Once the Beast was out of sight, Sly grabbed the treasure key and exited the area.
I had to overcome some personal demons to get the answer to this one. Try 4-4-4.