The electronic properties of gold-silver binary cluster anions (AunAgm-; 2 less than or equal ton+m less than or equal to4) were studied by using photoelectron spectroscopy (PES) with a magnetic-bottle-type electron spectrometer. The AunAgm- cluster anions were generated by a laser vaporization of a gold-silver alloy rod. For triatomic AunAg3-n clusters, monotonous increases of electron affinity (EA) were observed by the replacement of Ag atom by Au atom. In contrast, the change in EA of tetratomic AunAg4-n clusters was irregular; for n=0-2 their EAs are almost the same around 1.5 eV, whereas for n=3,4 they increase to around 2.7 eV. This irregularity in EA of AunAg4-n can be attributed to the contribution of an ionic bonding. For Au1Ag3- and Au2Ag2-, moreover, the PES spectra show two components whose intensity ratio depends on cluster source conditions, showing that two isomers should coexist at these two compositions.