You need to defeat the Mhasmeah enemies in that area to get the Purple Fuel items.
Each time you press a button down it consumes 1 Purple fuel.
Walking over a button after it was pressed will pull it up and it'll cost another fuel to press it again, so avoid walking over pressed buttons.
After all four buttons are pressed down a special boss enemy will appear in the middle of the area. There is no text or sound to inform you about her arrival when all buttons are down. Once the boss is defeated all buttons will be up again, and you need another 4 fuel to press them all and summon the boss again.