2 unit is never counted.
only 4 unit and 3 unit
4 unit becomes 2 units
3 unit becomes 2 units
which is good, because if you're pro at maths.. you double the effects of super scaling from 3 unit + 4 unit.
also, the exams are on the same time. and most 4 unit students prob won't study for the 2 unit exams if there was one