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A high-speed (400 Gb/s) and long-reach (180 m) four-level pulse amplitude modulation (PAM4) optical wireless communication system employing Mach-Zehnder modulator (MZM)-optoelectronic oscillator (OEO) broadband light source (BLS) and doublet lenses is proposed. Analytic results show that the data rate is markedly improved by PAM4 modulation and MZM-OEO BLS with multiple wavelengths, and the free-space transmission distance is considerably increased by a couple of doublet lenses. With the support of PAM4 modulation, MZM-OEO BLS, and doublet lenses, good bit error rate performance and clear PAM4/nonreturn-to-zero eye diagram are achieved at a 400 Gb/s/180 m operation. The proposed system is presented to be an eminent one not only as a result of its evolution in free-space optical (FSO) links, but also on account of its high-speed and long-reach characteristics in FSO links.