With four terminals you have a SPST relay, normally open, closed when energized. If you've identified the actuating coil contacts (85 & 86), then the other two are your load contacts (30 & 87) and they are a simple switch. You can hook your 12V battery power to either one, and run your output to the headlights from the other. Here's a link that someone posted previously (streetworks??) that explains the mysteries of relays:
http://www.watsons-streetworks.com/current_topic.html