The Big Four (ROTBTD)

The Big Four is the poly ship between Jack Frost, Merida, Rapunzel and Hiccup from the Rise of the Guardians, Brave, Tangled and How to Train Your Dragon fandoms.

Canon
The Big Four, also known as Rise of the Brave Tangled Dragons, is a four way crossover between two Disney films and two Dreamworks films that began during the ending of 2012 and continued to flourish by the time 2013 came around. The four films associated within the fandom are: Rise of the Guardians, Brave, Tangled and How to Train Your Dragon. Not only that, but the main characters who are focused within this crossover also happen to be the main protagonist of each featured film aka, Jack Frost, Merida, Rapunzel and Hiccup Horrendous Haddock III. Merida and Rapunzel are the only ones in the group who had a canon interaction during the Ralph Breaks The Internet movie, in which both characters appeared.

Although just because the main focus happens to be on the four main protagonists, does not mean that other characters from each featured film don't appear. Rather, characters such as The Guardians and Flynn Rider, do still make countless appearances as well as do the antagonists of each film. In fact, Pitch Black himself, has been shown to be the number one antagonist within this crossover and it’s usually because of his actions that Jack, Merida, Rapunzel and Hiccup tend to band and work together in order to defeat him.

Fanon
The Big Four is a very popular ship and considered a significant part of 2010s fandom spaces. The origin of this ship is unclear as no one actually knows who came up with the idea to create this crossover ship or why they decided to put these four characters together. However, fans have come up with many theories as to why this fandom came to be.

At the time, Rapunzel and Merida were the two latest main characters of their respective Disney movies. And since they were both part of the Disney Revival Era, fans started making crossover fanworks of the two and even shipping them. On the other hand, Jack Frost and Hiccup were the two latest Dreamworks heroes. How to Train Your Dragon became extremely popular the first moment the film came out and so Hiccup alone was beloved by many. In Jack's case, despite the fact that the Rise of the Guardians movie did not perform well at the box office, his character became quite popular within fandom spaces, so many fans decided to add these two characters to the group. During the time, many fans were already crossing over these two characters within their own crossover universe, which was known as How to Train Your Guardian, and even shipping them romantically.

Fans have also noticed some similares regarding the characters' universes. For example, fans noticed that the word “viking” is mentioned in the Brave movie. This led fans to infer that Merida’s time period is roughly the same as Hiccups, which made them think that a crossover between these two characters could be possible, despite the fact that they are from two different rival companies.

Fans also theorized that these four match together because they all happen to be teenagers who had to face similar issues. Jack wanted to be seen because he was tired of being invisible after so many years. Merida wanted to break free from the traditions her mother tried to put on her and make her own fate. Rapunzel wanted to leave her tower so that she could finally see the lanterns that came out on her birthday. And Hiccup wanted to be accepted by his village and prove that he can be a viking like everyone else is.

They all had something they wanted to do when their film first began, but then as their films progressed, the goal they all originally wanted suddenly changed. They all realized that what they wanted wasn’t what they thought, or rather, it wasn’t something they needed. So, while this also happens to a lot of characters from different films, fans found many similarities between these four characters' stories, which made the ship even more popular as a poly ship.