  We examine entanglement dynamics via concurrence among four two-state systems
labeled $A, ~a, ~B, ~b$. The four systems are arranged on an addressable
"lattice" in such a way that $A$ and $a$ at one location labeled $Aa$ can
interact with each other via excitation exchange, and the same for $B$ and $b$
at location $Bb$. The $Aa$ location is prepared entangled with the $Bb$
location, but their mutual complete isolation prevents interaction in the
interval between actions of an external addressing agent. There are six
pairwise concurrences on the lattice, and we follow their evolution in the
interval between external actions. We show how entanglement evolves and may
exhibit the non-analytic effect termed entanglement sudden death (ESD), with
periodic recovery. These loss and gain processes may be interpreted as
entanglement transfer between the subsystems.
