  Let $G$ be a simple graph with order $n$ and adjacency matrix
$\mathbf{A}(G)$. Let $\phi(G; \lambda)=\det(\lambda
I-\mathbf{A}(G))=\sum_{i=0}^n\mathbf{a}_i(G)\lambda^{n-i}$ be the
characteristic polynomial of $G$, where $\mathbf{a}_i(G)$ is called the $i$-th
adjacency coefficient of $G$. Denote by $\mathfrak{B}_{n,m}$ the set of all
connected graphs having $n$ vertices and $m$ edges. A bipartite graph $G$ is
referred as bipartite optimal if $$\mathbf{a}_4(G)=min\{\mathbf{a}_4(H)|H\in
\mathfrak{B}_{n,m}\}.$$ The value $min\{\mathbf{a}_4(H)|H\in
\mathfrak{B}_{n,m}\}$ is called the minimal $4$-Sachs number in
$\mathfrak{B}_{n,m}$, denoted by $\bar{\mathbf{a}}_4(\mathfrak{B}_{n,m})$.
\vspace{2mm} For any given integer pair $(n,m)$, we in this paper investigate
the bipartite optimal graphs. Firstly, we show that each bipartite optimal
graph is a difference graph (see Theorem 10). Then we deduce some structural
properties on bipartite optimal graphs. As applications of those properties, we
determine all bipartite optimal $(n,m)$-graphs together with the corresponding
minimal $4$-Sachs number for $n\ge 5$ and $n-1\le m\le 3(n-3)$. Finally, we
express the problem of computing the minimal $4$-Sachs number as a class of
combinatorial optimization problem, which relates to the partitions of positive
integers.
