The "4 Traveler Fiesta" is where you complete a single story run with randomly chosen characters. This is how it's done:

Copy and paste this into the list randomizer at https://www.random.org/lists/ and click "Randomize".

Ophilia
Cyrus
Tressa
Olberic
Primrose
Alfyn
Therion
H'aanit

The 1st character in the list is the character you start with. The 2nd, 3rd and 4th characters are the other characters you can get. You can get the others in any order.

The goal is to complete the first character's chapter 4 and trigger the credits (the same as a normal single story run).