Suppose $K$ is a $n$-knot, $n\geq 2$, which bounds two different ribbon disks $D_1, D_2$. These ribbon disks induce unique ribbon $(n+1)$-knots $K_1, K_2$ respectively. Is it known whether $K_1$ and $K_2$ are necessarily isotopic (obviously they must be if the ribbon disks are stably equivalent)?
For the $n=1$ case Nakanishi and Nakagawa have counterexamples to this in On Ribbon Knots, Math. Sem. Notes Kobe Univ. 5, 1982. However, their examples don't seem generalize in an obvious way to higher dimensional ribbon knots.
