This paper uses some ideas
from 3-dimensional topology to study knots in S4. We show that the Poincaré
conjecture implies the existence of a non-fibered knot whose complement fibers
homotopically. In a different direction, we show that Gromov’s norm is an
obstruction to a knot having a Seifert surface made out of Seifert fibered spaces, and
hence to being ribbon. We also prove that any 3-manifold is invertibly homology
cobordant to a hyperbolic 3-manifold, so that every knot in S4 has a hyperbolic
Seifert surface.