4. Given 1987 sets, each set has exactly 45 elements, and the union of any two sets has exactly 89 elements, the number of elements in the union of these 1987 sets is ( ).
A. 87429
B. 89415
C. 89326
D. 89504
4. A.

Among these 1987 sets, the intersection of any two sets contains exactly one element; furthermore, among these 1987 sets, take a set $A$, which contains an element $a$, and $a$ appears in at least 46 sets outside of $A$, and $a$ is the only common element among these 1987 sets. Therefore, the number of elements in the union of these 1987 sets is 87429.