One important characteristic of a parallelogram is that the diagonals of a parallelogram bisect each other.
In the given picture, MO and PN are the two diagonals of the parallelogram and they intersect at point A.
This means that, point A divides each diagonal equally.
Therefore:
AO = (1/2) MO
4 = (1/2) MO
MO = 4*2
MO = 8 units