It's a ruse...don't worry - 4 years. No extra. I asked Jane Sutter point blank, and she gave me some BS answer that just bounced off me.
The "real" answer they might give is that it is 5 years to satisfy the UK concerns. So as to not scare off US applicants, it's 4 years.
Oh, I remember - Jane Sutter said it had something to do with financial aid - that you needed 20 week semesters or something like that - she said, "You don't want to be in school for 10 months straight, do you" or the like....funny, that's just how it turned out. Our whole program is 156 weeks (now they say 157 - don't know where that other week came from), so it's the same as everywhere else.
My own logical sense is that the "5 school years in 4 calendar years" is, logically - $$$$.