  A signed graph $(G, \Sigma)$ is a graph $G$ and a subset $\Sigma$ of its
edges which corresponds to an assignment of signs to the edges: edges in
$\Sigma$ are negative while edges not in $\Sigma$ are positive. A closed walk
of a signed graph is balanced if the product of the signs of its edges
(repetitions included) is positive, and unbalanced otherwise. The
unbalanced-girth of a signed graph is the length of a shortest unbalanced
closed walk (if such a walk exists). A homomorphism of $(G,\Sigma)$ to
$(H,\Pi)$ is a homomorphism of $G$ to $H$ which preserves the balance of closed
walks.
  In this work, given a signed bipartite graph $(B, \Pi)$ of unbalanced-girth
$2k$, we give a necessary and sufficient condition for $(B, \Pi)$ to admit a
homomorphism from any signed bipartite graph of unbalanced-girth at least $2k$
whose underlying graph is $K_4$-minor-free. The condition can be checked in
polynomial time with respect to the order of $B$.
  Let $SPC(2k)$ be the signed bipartite graph on vertex set
$\mathbb{Z}_2^{2k-1}$ where vertices $u$ and $v$ are adjacent with a positive
edge if their difference is in $\{e_1,e_2, \ldots, e_{2k-1}\}$ (where the
$e_i$'s form the standard basis), and adjacent with a negative edge if their
difference is $J$ (that is, the all-1 vector). As an application of our work,
we prove that every signed bipartite $K_4$-minor-free graph of unbalanced-girth
$2k$ admits a homomorphism to $SPC(2k)$. This supports a conjecture of Guenin
claiming that every signed bipartite planar graph of unbalanced-girth $2k$
admits a homomorphism to $SPC(2k)$ (this would be an extension of the
four-color theorem).
  We also give an application of our work to edge-coloring $2k$-regular
$K_4$-minor-free multigraphs.
