In this manuscript, coherent polarization beam combining of four polarization-maintained fiber amplifiers is demonstrated. In each fiber amplifier, two cascaded fiber amplification stages are used to boost the output power to watt level. The compact and robust single-frequency dithering technique is employed in the active phase-locking of the four laser beams. When the system is in the closed loop, the phase noise can be suppressed effectively for long-time observation. The experimental results show that the combining efficiency of the whole system is as high as 95%.