To get everyone up to speed, Wave 4 will be arriving on 12th or 13th February 2020 (depending on where you live). It will include a new side story called “Cindered Shadows” and features new characters, classes, paralogues and activities etc.
The first thing to note is that Cindered Shadows is separate from the main game, with its own save files. To play it, you must choose the new “Side Story” option located between “Nintendo eShop” and “Copy”.
When creating a save file, players can choose the protagonist’s form and name, as well as select the difficulty (Normal or Hard) and mode (Classic or Casual). Just like when you begin a new save file for the main game.
In the main game, players followed a monthly routine where they taught and socialised with their students, while preparing for the end-of-month task. However, Cindered Shadows will have a different gameplay cycle (no details given).
Also, players will be provided with fixed team members and won’t be able to recruit other students. Furthermore, the maps will feature a lot of gimmicks and the overall difficulty is a bit higher than the main game.
(Not mentioned here, but by playing the side story, you can recruit new characters from the side story in the main game.)
The side story occurs near the start of Part 1 of the main game, just when the students are getting accustomed to academy life. It begins when the protagonist, the three House Leaders and a few students follow a suspicious figure into the monastery’s basement.
Beyond the tunnels, they discover a town called “Abyss”, oft spoken of in rumours. Abyss is the last refuge for those who wish to escape society. For some reason, the Church of Seiros has reluctantly allowed Abyss and its denizens to exist.
The “Ashen Wolves” are the secret 4th house of Garreg Mach. Members of this house originally attended the Officer’s Academy, but because of various circumstances, they ended up in Abyss, where the surface laws do not apply.
Yuri (VA: Junya Enoki)
Constance (VA: Sarah Emi Bridcutt)
Balthus (VA: Subaru Kimura)
Hapi (VA: Sachika Misawa)
Recently, Abyss has frequently been attacked by enigmatic invaders from the surface. Even though there’s a “house”, there’s nobody to guide them. Nonetheless, the protagonist decides to assist Abyss and those fighting to protect its vulnerable dwellers.
Update: Corrected the above passage in light of a recently added character profile.
Oddly, there’s one new screenshot that I don’t think was present in the trailer. It shows Yuri triggering a Mystery Crest. If you compare it with the known Crests, you’ll find it’s the Crest of Aubin. This also matches with the data-mined information.