Answer
$15$
Work Step by Step
The number of unordered sets that contain four items chosen from six equals to the number of combinations of 6 items taken 4 at a time : $C(6,4)=\frac{6!}{4!(6-4)!}=\frac{6!}{4!*2!}=\frac{6*5*4*3*2*1}{4*3*2*1*2*1}=\frac{6*5}{2}=15$
