Isotopy of orientation-preserving diffeomorphisms of the 4-sphere

Let $f\colon S^4\to S^4$ be an orientation-preserving self-diffeomorphism. Smale conjecture, special case. Every orientation-preserving self-diffeomorphism of $S^4$ is smoothly isotopic to the identity. This is presented as an important case that remains open, concerning the mapping class group of the 4-sphere.