Found all four locations today. Each switch is INSIDE the large quest circle on the map.
From the entrance to Caustic Caverns: 1. Under the acid-soaked railway - Go right until you see the area with the large Crystalisks and acid pools. When you see large columns (supporting long fallen down railtracks) go to the third one out from the cliff and you'll find the switch at its base. 2. Warehouse by the shore - In the northern part of the map, you'll come to the area with many small larvae spawning. The switch is by the shore in this area. 3. In the shadow of the digger - In the western part of the map, look up and you'll see a rotating digger (like ones you would find at a coal mine). Go inside the semi enclosed warehouses and you'll find the switch on a wall. 4. In the bloody Dahl 6 - This area is right next to the start of the area but is closed off until you make a full circle of the map. When you pass through the area with all the Spiderlings, you'll find a door leading to the Dahl 6 Core area. It's only a small room with a large ladder leading up and out of the room. The switch in on one of the walls.