The 4th oracle is at the end of the wall, jumping puzzle in the green room. However rather than doing the full puzzle, you can stop once you reach the halfway point on the wall opposite from where you entered the room/exit ther room. Once you've reached this spot, if you ADS and look at the wall, across from you, a small white box should be visible. This is where you'd go to exit the room is you did the jumping puzzle rather than the shortcut. If you have a rocket, preferably a high impact frame, you can shoot rockets at this box and damage/destroy the oracle. Make sure you aim for the top center or left corner of the box. I'd recommend having lasting impressions on your rocket. If done right, you'll see damage numbers pop up. You may be able to shoot the oracle from this spot with a sniper or scout as well if you aim for the top center.