question: We investigated Cu4On - (n = 1-4) clusters through a synergetic combination of mass-selected anion photoelectron spectroscopy and density functional theory calculations. It is found that the most stable structure of Cu4O- is an irregular planar pentagon with a Cs symmetry. Those of Cu4O2 - and Cu4O3 - are non-planar structures with a Cs symmetry. The global minimum geometry of Cu4O4 - is a D4h symmetric quasi-square eight-membered ring with Cu-O bond lengths of ∼1.78 Å. The molecular orbital analyses suggest that Cu4O4 - has a large highest occupied molecular orbital and lowest unoccupied molecular orbital gap. The chemical bonding analyses and the calculations of the magnetically induced current density, and NICS(0) and NICS(1) values indicate that the D4h structure of Cu4O4 - is very stable and it has some aromaticity.