There are electives that you must take but can be exempted if you have credit from other College or university. In my son's case, he was exempted for all the electives therefore he didn't have to take 5 to 6 courses each semester like his classmates.
However, most of the upper years courses required 'pre-requisite' courses e.g. ABC 200 requires successfully completed ABC 100 before you can register. Therefore a 4 semesters program normally takes 4 semesters.