The development history, key techniques and applications of four-mode differential laser gyros (FMDLGs) are summarized so as to provide some reference for domestic development of FMDLG. The keys to improve the performance of FMDLG are concluded according to investigation on materials related to FMDLG, including fundamental research, technical improvement, optimization design for cavity, electronic system design and error compensation technique. In order to fully realize the potentials of FMDLG, each key aspect should be taken into account elaborately.