Seifert surfaces of knots in \(S^ 4\) (Q806111)

In this nice paper the author studies knots in \(S^ 4\). He proves that the Poincaré conjecture implies that there is a non-fibered knot in \(S^ 4\) whose complement fibers homotopically. He also exhibits the Gromov's norm as an obstruction to finding a Seifert surface which is a Seifert fibered space. So, in particular, this norm is an obstruction the knot being a ribbon. He shows that every knot admits a Seifert surface which is a hyperbolic manifold, by proving that any 3-manifold is invertibly homology cobordant to a hyperbolic 3-manifold.