Some classes of linear codes over the ring $$\mathbb {Z}_4+v\mathbb {Z}_4$$Z4+vZ4 with $$v^2=v$$v2=v are considered. Construction of Euclidean formally self-dual codes and unimodular complex lattices from self-dual codes over $$\mathbb {Z}_4+v\mathbb {Z}_4$$Z4+vZ4 are studied. Structural properties of cyclic codes and quadratic residue codes are also considered. Finally, some good and new $$\mathbb {Z}_4$$Z4-linear codes are constructed from linear codes over $$\mathbb {Z}_4+v\mathbb {Z}_4$$Z4+vZ4.