Pick 2 from 10 = 45 different ways
Pick 2 from 6 = 15 different ways
So 15 of 45 possible sets of 8 authors contain all 4 you want
ie 2/3 chance one or more will be dropped.
Working from that original set of 45 you can also figure out which ones remove 2 authors you chose (2 of 4 = 6) of which 2/3 of combinations make both your essays useless.
So it's 41/45 that you'll get at least one useful essay, 1/3 that you'll get 2.
Pick 2 from 6 = 15 different ways
So 15 of 45 possible sets of 8 authors contain all 4 you want
ie 2/3 chance one or more will be dropped.
Working from that original set of 45 you can also figure out which ones remove 2 authors you chose (2 of 4 = 6) of which 2/3 of combinations make both your essays useless.
So it's 41/45 that you'll get at least one useful essay, 1/3 that you'll get 2.