You can use a power resistor of about 250 ohms accross the output. While not a direct short, it will prevent the flyback effect of an open output that can cause voltage peaks that can burn holes in the winding insulation.
The way I wire output jacks is to use a standard jack for eight ohms and a "transfer" jack for the four ohm jack. When you plug something in to the four ohm jack, both jacks are then connected to the four ohm tap. This way it allows use of a single eight ohm cab in the eight ohm jack, or two eight ohm cabs into each jack, or a single four ohm cab in the four ohm jack.
Sorry I don't have a diagram. Go to one of the big electronic suppliers and look at the Switchcraft page. They will have a diagram for the transfer jack.