Depending on the weight of the briskets of course, but if you are talking about 36 - 40 lbs of uncooked brisket you will have enough to feed 70 people or everyone gets 10 + Oz of brisket each.
So 4 briskets may be plenty, especially if they have something else there as well. If it is brisket only and they are all BIG eaters you should still be ok, as most folks can't tuck it away like that without some extra pain.
So 4 briskets may be plenty, especially if they have something else there as well. If it is brisket only and they are all BIG eaters you should still be ok, as most folks can't tuck it away like that without some extra pain.