The Boston Globe investigates:
The deferments for Mormon missionaries became increasingly controversial in the late 1960s, especially in Utah, leading the Mormon Church and the government to limit the number of church missionaries who could put off their military service. That agreement called for each church ward, or church district, to designate one male every six months to be exempted from potential duty for the duration of his missionary work.
Romney’s home state was Michigan, making his 4-D exemption as a missionary all but automatic because of the relatively small number of Mormon missionaries from that state.
The Dish 