The 6 and 5 year courses are basically undergraduate courses, with students entering directly from high school. Therefore, usually they incorporate more basic science work in the first couple years (ie. biology, chemistry, physics). The US 4-yr programs are graduate programs so you start learning specific medically-specific subjects from the start because it is assumed and expected that you have had previous college-level experience (ie. incorporated into a previous undergraduate degree) in core subjects (biology, chemistry, organic chemistry, physics) that lead to better understanding of the medical subjects.
There are pros and cons of completing an undergraduate degree before medicine or not. They have been discussed in other threads extensively.
If someone from the US with a previous undergrad degree chooses to enter a 5 or 6 year UK/rest of world program, they may be eligible to skip years.
