The tetraoxide clusters with stoichiometry MO4, and the structural isomers with side-on and end-on bonded dioxygen, are studied by DFT with the B1LYP functional. Diperoxides M(O-2)(2) are the most stable clusters at the beginning (Sc, Ti) and at the end of the row (Co-Cu), the latter being planar. For V, Cr, and Mn, the dioxoperoxides O2M(O-2) are the most stable isomers. Low-spin states are dominant for the nonplanar diperoxides M(O-2)(2) and dioxoperoxides O2M(O-2), and the local magnetic moment at the metal cations is small. The local charge on the metal cation center is higher in the diperoxides of Sc and Ti; it drops significantly in the dioxoperoxides of V and Cr. The iron dioxosuperoxide in the (3)A" state, which contains end-on bonded dioxygen, OOFeO2, is an exception with higher charge on Fe. In the planar diperoxides of Co, Ni, and Cu, oxygen-to-metal charge transfer is significant, and the local charge on the metal cation is close to 1. In all tetraoxygen clusters of the 3d elements, the cation center remains strongly electrophilic and interacts with Ar atoms from the inert-gas matrix, where the clusters are trapped for IR spectral studies. Significant frequency shifts in the matrix are found for the dioxoperoxide of vanadium, O2V(O-2), the dioxosuperoxide of iron, OOFeO2, and the nickel diperoxide, Ni(O-2)(2).