Driven by continuously growing mobile traffic, line rate of digital mobile fronthaul (MFH) network keeps surging. 4-level pulse amplitude modulation (PAM4) is a promising format to provide such high data capacity, due to its bandwidth and cost efficiency. In this paper, we propose an improved method to reduce mobile signal distortion caused by bit error of optical transmission. The concept comes from the characteristic that high order sample bit error induces far severer performance degradation to radio signal than low order one. In the solution, high order sample bits and low order sample bits are interleaved, so that they are respectively mapped to first bit (1stb) and second bit (2ndb) of PAM4 symbol. On the other hand, amplitude levels of PAM4 are set unequal to broaden the "middle eye", levering accuracy of 1stb. Hence, the total fidelity of mobile signal is enhanced. The feasibility is confirmed both theoretically and experimentally. The investigation is based on two typical digital systems, i.e., 16-bit uniform quantizing and 8-bit nonuniform quantizing. Experimental results indicate that, EVM of LTE-A like radio signal decreases by up to 13dB in uniform quantizing system, and by up to 5dB in nonuniform quantizing system, compared with conventional equally-spaced PAM4.