4 pin relays for headlights

In a word, Google. Wiring diagrams for spdt relays and headlight circuits are all over the web. More important is the quality of relays and even more so is their connectors. Some very cheap junk can be had if you want to go pennies, and ebay, and mailbox. You would be much better off to go to a parts yard and clip some quality components from a factory harness. You'll find either actual Bosch relays with weather skirts or harness connectors with weather boots. Some of the better pieces I've seen came from near the radiator and headlight doors of 86 through 91 Honda Accord. Not easy to get out of a wreck but worth the trouble. Alternative is a enclosure like more modern vehicles have. Typical generic naked components poked under the battery tray/wherever will eventually be corroded just like bulkhead/ battery/ other connections.