The position vector of the midpoint of the vector joining two points can be found by taking the average of the corresponding coordinates of the two points. In this case, the midpoint of the vector joining P(2, 3, 4) and Q(4, 1, -2) can be found by taking the average of the x-coordinates, y-coordinates, and z-coordinates. Therefore, the position vector of the midpoint is [(2+4)/2, (3+1)/2, (4+(-2))/2] = [3, 2, 1].