Mine says four years, as well! I don't think it's a mistake, based on the stipend quoted in the award package (it seems like the annual salary would be much too high if the net stipend funds were distributed across three - not four - years). Also, offering longer-term fellowships would seem to be consistent with the DoD offering fewer fellowships this year.
Congrats to all who received an award today, and good luck to those on the waiting list!