Mutual locking of four Gaussian beams with prescribed relative amplitu
des and phases in a resonator ended by a phase-conjugating mirror is s
hown. Locking is performed thanks to a pair of mutually matched spatia
l filters located in the Fourier planes of the resonator. Experimental
results are obtained in a cw-oscillator using a barium titanate cryst
al both as phase-conjugating mirror and amplifying medium.