Perhaps the four heroes were somehow able to communicate with each other. Let's assume they had a sense of humor. One day, in their chests, they were like:
Hero 1: I'm bored! How long have we been trapped in these chests?!
Hero 2: I lost count at 300 years.
Hero 3: We've been stuck here for 500 years!
Hero 4: What?! If we have to wait another 500 years, we should mess with the opener's head! Like, say the special skills we grant him/her are terrible curses?
All: Yeaah!