We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa4- and NaIn4-. Photoelectron spectra of the anions were obtained and compared with ab initio calculations. We found that the ground state of the two anions contains a square planar dianion interacting with a Na+ cation. The Ga-4(2-) and In-4(2-) dianions both possess two delocalized pi electrons and are considered to be aromatic, similar to that recently found in Al-4(2-). Using calculations for a model compound, we showed that a recently synthesized Ga-4-organometallic compound also contains an aromatic -Ga-4(2-)-unit, analogous to the gaseous clusters.