The four-mode differential laser gyros (FMDLGs) would have better performance when operating at the point with equal scale factors for left circularly polarized (LCP) and right circularly polarized (RCP) gyros. In order to find the optimal operating point, expressions for FMDLG beat frequency are analyzed. Theoretical study shows that the sum frequency as a function of piezoelectric transducer (PZT) scanning voltage is quadratic and the scale factors of the LCP and RCP gyros are equal when the quadratic function is at its turning point. Therefore, theoretical optimal operating point can be obtained by recording beat frequency of the FMDLG while scanning the PZT voltage. And actual optimal operating point can be obtained by testing the FMDLG at different operating point. Experimental results give an approximate coincidence of theoretical and actual optimal operating point with maximum error of 2 Hz, which shows a reference method to choose optimal operating point for FMDLGs.