Quantum chemical calculations suggest that group 4 tetra-azides M(N-3)(4), where M = Ti, Zr, Hf, and Th, are stable species. They present a unique structural feature; namely, the M-N-N-N fragments are linear. These species are energetically more stable than the corresponding isomers with general formula eta(5)-N-5-M-eta(7)-N-7, and the Th species, Th(N-3)(4), is the most stable of all. Possible mixed nitride azides NMN3 were also investigated.