  Lattice coding for the Gaussian wiretap channel is considered, where the goal
is to ensure reliable communication between two authorized parties while
preventing an eavesdropper from learning the transmitted messages. Recently, a
measure called secrecy gain was proposed as a design criterion to quantify the
secrecy-goodness of the applied lattice code. In this paper, the theta series
of the so-called formally unimodular lattices obtained by Construction
$\text{A}_4$ from codes over $\mathbb{Z}_4$ is derived, and we provide a
universal approach to determine their secrecy gains. Initial results indicate
that Construction $\text{A}_4$ lattices can achieve a higher secrecy gain than
the best-known formally unimodular lattices from the literature. Furthermore, a
new code construction of formally self-dual $\mathbb{Z}_4$-linear codes is
presented.
