As I've finished building and testing all four of the fights, I figured now would be a good time to explain what they'll be like for anyone interested.
These four fights are designed as "raid boss" style encounters. ("challenge bosses" just sounds better in this context) They are meant to be attempted close to starting the third act, when your party is filled out and you've done as much of the other content in the game as is available. Each of them are built around a specific team composition, they have various phases to run through, and are the only fights in the game that won't let you use the "cheat" weapons and character from the secret room. You are expected to lose against them a few times until you figure out the exact party comp you're meant to be using, so save before actually interacting with them. Then it's like any other dungeon/raid boss in that you need to execute the correct sequence of skills to counter each phase. Once you've figured out what you're supposed to do they're not overly hard, as I tuned them down a bit to make sure they aren't as punishing as they were when I first built them, but they still require attention to detail and actually learning the fights.
The areas to fight these bosses aren't marked on the world map like every other location to prevent players from stumbling upon them while exploring and aren't prepared for the fights. As they are balanced with the expectation of specific team comps you will need to collect every companion in the game and complete any of their associated personal quests to get both their gear set and their signature ability as several of the fights require those abilities to progress or flat out win the battle. Usually things like this are solved fairly quickly so I'm not going to spoil the fights, however if in a few months people are still stuck on them I'll make sure to provide a walk-through of the fights so you can at least experience how they're meant to be played.