It's Michael Berry (Dan/Dr. Madden), Jessica Phillips (Diana), Meghann Fahy (Natalie) and Tim Young (Gabe/Henry)
Michael Berry went on for both Dan and Dr. Madden at Arena, Jessica Phillips went on once at 2nd Stage for Diana, and Tim Young went on a few times as Gabe at Arena (but I don't think he went on as Henry). I don't think Meghann has ever been on as Natalie.
Now what would you say if today I started over?
Without a thing but this taped together four leaf clover
And I'll pretend like everything is already alright
And I'll run toward the sun till the castle's out of sight