In [KM63], Kervaire and Milnor introduced the group of homotopy spheres. Its elements are h-cobordism classes of smooth homotopy $n$-spheres under the summation induced by connected sum. Further, the trivial element is $S^n$ and this group is denoted by $\Theta^n$.
They proved that $\Theta^n$ is finite unless $n=3$, in particular $\Theta^4$ is trivial.
This should be an ambiguous question but I wonder this provides a positive clue for the smooth Poincaré conjecture in dimension 4.