  We apply mapping class group techniques and trisections to study intersection
forms of smooth 4-manifolds. Johnson defined a well-known homomorphism from the
Torelli group of a compact surface. Morita later showed that every homology
3-sphere can be obtained from the standard Heegaard decomposition of $S^3$ by
regluing according to a map in the kernel of this homomorphism. We prove an
analogous result for trisections of 4-manifolds. Specifically, if $X$ and $Y$
admit handle decompositions without 1- or 3-handles and have isomorphic
intersection forms, then a trisection of $Y$ can be obtained from a trisection
of $X$ by cutting and regluing by an element of the Johnson kernel. We also
describe how invariants of homology 3-spheres can be applied, via this result,
to obstruct intersection forms of smooth 4-manifolds. As an application, we use
the Casson invariant to recover Rohlin's Theorem on the signature of spin
4-manifolds.
