In order to prove that the mathematics of four observers works in determination of event location, I suggest that the following experiment could be done in a stadium. Given that observers share a common time and awareness of their relative coordination location in space...
Select any three points (3 points define a plane) from the stadium floor. Select any seat from the stadium to be the fourth observer location (therefore point 4 is not that same plane). Next pick any spot as the origin of an event.
Given the knowledge of these 4 observer positions and
1 which observer is closest (first to receive the event horizon from that event)
2 the length interval between the other three observers and the closest observer will allow for determination both the time and position of the event origin.