At the switchhouse, study the trail plan. Switch 4 comes after switch 1. Adjust switch 1 so that you will pass by switch 4.
Now back into the car for another round. You should pass switch 1 and then arrive at switch 4 after a while. Stop the car there and go to the switch. Walk near the switch until Indy mentions that it would need some grease. Now use the oil can from your inventory, Indy will knee down and glubb the oil into the switch.
Again, back into the car. Ride until you're back at the switch house, there you'll be able to move the lever for switch 4.