The two should be bonded together in the main panel. The only time you
will need 4 wires for a 240 volt circuit is if you also have some
component of the circuit feeding a 120 volt load. Then you will have
neutral current as well as 2 hots and a ground. An example of this
would be a 240 volt stove with a 120 volt convenience outlet on it, or
a subpanel with 120 and 240 volt circuits in it.

Stretch