I think you have to differentiate between the offshore schools and UWI, which is designed to train doctors for the Caribbean. UWI (in Trinidad and Jamaica) is I believe 6 years because the English Caribbean follows the British education system and people enter medical school after A-levels, as opposed to after finishing a first degree. Keep in mind A levels are basically at the level of early university courses. Most British med schools are like this and its only recently that a few programs of shorter length have been developed there.
From what I know, the only offshore schools that are over 4 years take people without the usual prereq's, so these students need time to complete them or do other coursework.
