3 people (Wigan Smith and 2 others)
This solution only applies because Wigan Smith has no moral character.
As each person can carry five days rations for 1 person , and that the total journey will require 8 days (4 days there, overnight stay, then 4 days back), then the journey would seem impossible. However, Smith and 2 companions could travel 2 days and have 3 rations each. If one companion "Mysteriously died" that night, Smith and his other companion would have 4.5 rations each. At the end of the fourth day they would have 2.5 ration each. If Smith's last companion "Mysteriously died" that night, He would have 5 rations, which would be more then enough to stay an extra day and return to the coast.
Today's brain teaser courtesy of Braingle.com.