Let's see how the reports can be distributed...

1. ALL reports assigned to one secretary (0,0,4)

3!/2! = 3 ways

2. Three reports assigned to one secretary (0,1,3)

3! = 6 ways

3. Two reports assigned to one secretary (1,1,2)

3!/2! = 3 ways

Therefore, probability of at least one report per secretary is 3/9 = 1/3