  Let p and q be distinct integers greater than one. We show that the
2-component pretzel link P(p,q,-p,-q) is not slice, even though it has a ribbon
mutant, by using 3-fold branched covers and an obstruction based on Donaldson's
diagonalization theorem. As a consequence, we prove the slice-ribbon conjecture
for 4-stranded 2-component pretzel links.
