I understand what you're saying now.
I'd consider Sleep Mastery for the fourth slot - it's kind of Endurance related, and would work well for an experienced Borderer. The feat "Tough as Nails" from Pirate Isles has Diehard as a prereq, so it may match better. Carouser is also sort of Endurance related, but may not suit Hyperboreans (I don't have the BB&N book).