  Recently, Chau et al. [Phys. Rev. A 95, 022311 (2017)] reported a
quantum-key-distribution (QKD) scheme using four-dimensional qudits.
Surprisingly, as a function of the bit error rate of the raw key, the secret
key rate of this scheme is equal to that of the (qubit-based) six-state scheme
under one-way classical communication using ideal apparatus in the limit of
arbitrarily long raw key length. Here we explain why this is the case in spite
of the fact that these two schemes are not linearly related to each other. More
importantly, we find that in terms of the four-dimensional dit error rate of
the raw key, the Chau et al.'s scheme can tolerate up to 21.6% using one-way
classical communications, which is better than the Sheridan and Scarani's
scheme [Phys. Rev. A 82, 030301(R) (2010)]. In addition, we argue the
experimental advantages of the Chau et al. implementation over the standard
six-state scheme and report a corresponding proof-of-principle experiment using
passive basis selection with decoy states. We also compare our experiment with
the recent high secret key rate implementation of the Sheridan and Scarani's
scheme by Islam et al. [Sci. Adv. \text{3}, e1701491].
