  For embedded 2-spheres in a 4-manifold sharing the same embedded transverse
sphere homotopy implies isotopy, provided the ambient 4-manifold has no
$\BZ_2$-torsion in the fundamental group. This gives a generalization of the
classical light bulb trick to 4-dimensions, the uniqueness of spanning discs
for a simple closed curve in $S^4$ and $\pi_0(\Diff_0(S^2\times
D^2)/\Diff_0(B^4))=1$. In manifolds with $\BZ_2$-torsion, one surface can be
put into a normal form relative to the other.
