Given that $n(A \cup B) = 23$, $n(A \cap B) = 4$, $n(A) = 21$, and $n(A' \cup B') = 51$, determine the number of elements in each of the disjoint regions labeled with Roman numerals in the Venn diagram to the right.
There are element(s) in region I, element(s) in region II, element(s) in region III, and element(s) in region IV.