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We report for the first time on the feasibility of quaternary (4-ary) pulse amplitude modulation (4-PAM) for both downstream (DS) and upstream (US) directions in a full-duplex passive optical network (PON) with wavelength reuse. The scheme is validated with a simple, low-cost, and colorless optical network unit (ONU) exploiting the all-optical carrier recovery scheme. The ONU employs a high-finesse optical filter with periodic frequency response for carrier extraction and data erasure of the DS signal. A cascade of two semiconductor optical amplifiers (SOAs) amplifies and modulates the recovered carrier with the 4-PAM US data-stream. The proposed scheme constitutes a migration scenario towards low-cost TDM/WDM PONs with currently deployed hardware equipment, ensuring full-duplex transmission up to 26 km, with higher spectral efficiencies and auspicious wavelength allocation.