In this work, a record output power of 4.6 kW linearly polarized and narrow-linewidth fiber amplifier based on an optimized fiber oscillator laser (FOL) seed was realized by employing a homemade polarization-maintaining Yb-doped fiber (PMYDF), corresponding to a slope efficiency of 79.5% and a 3 dB linewidth of 0.3452 nm. Through an effective strategy relying on decreasing the transmission fiber length from 200 m to 120 m and adding a chirped and tilted fiber Bragg grating (CTFBG), the stimulated Raman scattering (SRS) effects were well-suppressed. By applying the forward combiner with the interconnection between the pump arms into the MOPA system, the MI threshold is increased by more than 560 W and the slope efficiency of the upgraded MOPA system is boosted by 5%. During the experimental process of power amplification, the polarization extinction ratio (PER) remains higher than 15 dB, and a near-diffraction-limited output beam at the laser power of 2980 W was measured with the M2x = 1.314 and M2y = 1.311.