Connecting a 4 wire 240+120 volt hot tub to a 3 wire 240 volt circuit
If the hot tub is a 4 wire hookup it apparently needs a neutral for the controls to operate at 120 volts while the heater uses 240 volts.

So the short answer is: You don't hook it up to that wire.

You will need to run another wire if it is in conduit. White in color, the same size as the hot legs.

If it is a cable, then you will need to replace the cable unless the ground wire of the cable is insulated. Is the ground wire insulated?

Good luck!