Carbon/sulfur binary cluster anions with various compositions can be produced from laser vaporization of a sample mixed with sulfur and carbon powders in a 20:1 molar ratio. After mass-selection, their molecular formula was determined by collision-induced dissociation. The cluster anions consist of even carbon atoms only and their number of clustering sulfur atoms equals or exceeds that of carbon atoms. In this paper, we focus on mass spectrometry investigation and ab initio calculations on the cluster anions containing four carbon atoms, C4Sm- (4 less than or equal tom less than or equal to 10). Geometry of various isomeric structures of the clusters was optimized at the ROHF/6-31G* level and their energies were compared to find the most stable isomers. The structures of C4Sm- clusters computed in this paper were designed based on the structural features learned from the previous study on C2Sm- and the results of mass spectrometry experiments. Among the structures, four low-lying forms with two dangling sulfur atoms and two closed rings have the lowest energy. Based on the experimental and theoretical investigations, a ring shrinkage scheme is proposed to account for the dissociation process of the binary cluster anions observed in the experiment.