Treasure Hunters by James Patterson and Chris Grabenstein with Mark Shulman illustrated by Juliana Neufeld

Bick, Beck, Tommy and Storm have been home-schooled all of their lives as they travel the world with their treasure-hunting parents.
Recently their mother disappeared in Cyprus and then in the beginning of this book, their father goes overboard during a storm.  Now the siblings are orphans at sea.
As they finish up their father's current assignment, they encounter very interesting, nefarious individuals.   Whom can they trust?
 Together, with their talents, they overcome major obstacles.
Very exciting and entertaining!
I also really liked how the siblings treated each other (expect for Bick and Beck's famous twin tirades which always end with them being fine with each other/no resentment).
I won't recommend this series to my fourth grade son (see below).
Perfect series for middle school boys who are reluctant readers.
Guided Reading Level: T (fourth grade)
Grade Level Equivalent: 4.6 
"A hi-low book-will appeal to older readers but who are reading at a lower grade reading level".
Warnings/points to discuss: Tommy, the oldest brother age seventeen, is into girls and frequently checks them out.  He especially likes art with naked women. Violence with gun pointing.