The purpose of this paper is to prove that the symmetric fourth
power of a cusp form on ${\rm GL}(2)$, whose existence was proved
earlier by the first author, is cuspidal unless the corresponding
automorphic representation is of dihedral, tetrahedral, or octahedral
type. As a consequence, we prove a number of results toward the
Ramanujan-Petersson and Sato-Tate conjectures. In particular, we
establish the bound $q\sp {1/9}\sb v$ for unramified Hecke eigenvalues
of cusp forms on ${\rm GL}(2)$. Over an arbitrary number field, this
is the best bound available at present.