Using a 200 μm-core multimode optical fiber as the phase conjugator mirror, an LD-pumped Q-switched four-pass MOPA system operated on 100 Hz repetition rate and 20 ns pulse width has been experimental investigated in this paper. As a result, high quality laser beam with energy of 4.1 mJ and energy-stability less than 10% have been obtained when 4.6 mJ pump energy injected into fiber phase conjugator. The self-oscillating (SO) in amplifier and amplified spontaneous emission (ASE) has been validly suppressed. In additional, the pulse width of laser has been compressed to 4.7 ns. Due to the using of fiber phase conjugating mirror, the deformation formed by the thermal effect from the slab amplifier has been compensated after four passing. The output beam profile resumes the intensity distribution of the oscillator output beam.