  In this paper, we introduce a fundamental substructure of maximal caps in the
affine geometry $AG(4,3)$ that we call \emph{demicaps}. Demicaps provide a
direct link to particular partitions of $AG(4,3)$ into 4 maximal caps plus a
single point. The full collection of 36 maximal caps that are in exactly one
partition with a given cap $C$ can be expressed as unions of two disjoint
demicaps taken from a set of 12 demicaps; these 12 can also be found using
demicaps in $C$. The action of the affine group on these 36 maximal caps
includes actions related to the outer automorphisms of $S_6$.
