To summarise:
Solo monsters are very poorly done. So get four monsters (what the encounter system is roughly based around), and make them one monster.
So four health bars, one turn in the Initiative order per Health Bar, no bleed over damage between health bars, and recovering from all inflictions when a health bar is fully depleted (and no healing depleted health bars).
That's really interesting, might try it on a "sub boss" to see how that pans out.