Let \(F, G \in \mathbb {Z}[X, Y]\) be binary forms of degree \(\ge 3\), non-zero discriminant and with automorphism group isomorphic to \(D_4\). If \(F(\mathbb {Z}^2) = G(\mathbb {Z}^2)\), we show that F and G are \({\textrm{GL}}(2, \mathbb {Z})\)–equivalent.