  We address a continuous-variable quantum key distribution (CV-QKD) protocol
employing quaternary phase-shift-keying (QPSK) of coherent states and a
non-Gaussian measurement inspired by quantum receivers minimizing the error
probability in a quantum-state-discrimination scenario. We consider a pure-loss
quantum wiretap channel, in which a possible eavesdropper is limited to collect
the sole channel losses. We perform a characterization of state-discrimination
receivers and design an optimized receiver maximizing the asymptotic secure key
rate (SKR), namely the key-rate optimized receiver (KOR), comparing its
performance with respect to the pretty good measurement (PGM) and the
heterodyne-based protocol. We show that the KOR increases the SKR for
metropolitan-network distances. Finally, we also investigate the
implementations of feasible schemes, such as the displacement feed-forward
receiver, obtaining an increase in the SKR in particular regimes.
