Originally Posted by orlok
A "Quartet". For example, Lawrence Durrell's "Alexandria Quartet".
I was just coming back to suggest that. The Earthsea books by Ursula K Le Guin are also collected as the Earthsea Quartet.
All of these, though, surely suggest that the sequence is more-or-less complete. If you're adding more books, I'd suggest just adding Omnibus Volume 1 to the series title.