We report the first observation of an infrared (v 0 , 0 ~4150cm - 1 ) electronic band system arising from excitation of the ground state O 4 - ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge-resonance stabilized dimer ion.