The four live on the Isle of Skulls attached to Midworld and can only be approached through a ritual of events.
Anyone who wants to seek audience from the Horsemen, must cross the River Styx by way of Charon's ferry. Charon may try to cheat or trick them. If they can outwit Charon's trickery, they can approach the main gates.
At the main gates there is a great black dragon that must be bribed to allow entry. If a proper bribe or offering is successful, they may approach the front door.
At the door, they must knock three times and wait for the butler. The butler will ask names and purpose. This must be answered honestly or the penalty could be grave. The Horsemen don't like their time wasted by infidels.
Anyone who is directly summoned by the Horsemen or works for the Horsemen may bypass the tests.