Isomorphic graphs are graphs that contain the same number of connections (edges) and vertices, but they may differ in appearance. For two graphs to be isomorphic:
- The number of vertices must be the same.
- The number of edges must be the same.
- The adjacency relationships between the vertices must be identical.
However, the statement "The difference between the number of edges of any two isomorphic graphs must be one" is false because isomorphic graphs will always have the same number of edges.
Therefore, the correct answer is \( 4 \).