Five is an incident that happened to a hunting party that was hunting a Lao-Shan Lung.
How Did It Get The Name?
The incident got that name because, five Wyverians went to go hunt down a Lao-Shan Lung and only four came back. Two of these Wyverians were Kokoto Village's Chief and his fiancee. Sadly in the end, Kokoto Village Chief's fiancee died in the battle with the Elder Dragon though the monster was defeated. After that hunt, the four remaining Wyverians retired.
Sometime after that incident, the story of what happened became known across many lands. This lead to bad luck on hunts with five or more for hunters, with many hunts leading to the death of comrades in the end.
The Hunter's Guild Solution
In order to stop the deaths of more hunters, the Hunter's Guild added a rule that only allows four hunters to hunt together. Despite this rule by the Hunter's Guild there are a few exceptions to this rule, however. With particular quests, monsters, and emergency situations, hunters are able to hunt in more than fours.
The Hero of Kokoto - Story of the Five from the Hunting Life magazine from Monster Hunter Freedom.