Laser ablation of metal targets in front of condensing Ar/O-2 mixtures produced the strong antisymmetric fundamental of isolated O-4(-) at 953.5+/-0.3 cm(-1) in solid argon from the capture of ablated electrons by O-2 followed by the addition of another O-2 molecule. The sharp sextet mixed isotopic spectrum suggests a structure with two equivalent O-2 subunits and equivalent atomic positions in each subunit. DFT/UB3LYP calculations reveal doublet trans and rectangular structures with the latter more stable by 8.8 kcal/mol. Of more importance, the mixed oxygen isotopic spectrum calculated for the rectangular structure matches the observed spectrum much better than values calculated for the trans structure; thus, it is concluded that isolated O-4(-) probably has an approximately rectangular structure. It is further suggested that the earlier observed alkali metal M+O4- species have rectangular pyramidal structures and that the sharp 954+/-1 cm(-1) bands attributed to a different structural isomer are in fact due to the isolated anion owing to photoionization of the alkali metal atoms by radiation from the Nernst glower source of the grating spectrophotometer.