I have a 4 wire, 220V service coming into a 4 slot receptacle in my garage. I want to wire a 3 wire 220V 30 amp heater into the circuit by installing an Eaton double pole throw, single throw rocker switch. There is a double 30a breaker in my main panerl in the house for this circuit. How do I wire the switch? Is the neutral wire simply capped with a murette inside the switch (going from 4 wire to 3 wire)?

That is correct, assuming the heater requires a ground and not a neutral. Some of the heaters require a neutral because of a need for 120v for control circuits.

Are you sure this switch carries a Neutral connection. Normally switches are only used on the hot legs.