  We have developed a method to calculate a secret key rate of a
continuous-variable quantum-key-distribution scheme using four coherent states
and postselection for a general model of Gaussian attacks. We assume that the
transmission line and detection process are described by a pair of Gaussian
channels. In our analysis, while the loss and noise on the transmission line
are induced by an eavesdropper, Eve, who can replace the transmission line with
a lossless and noiseless optical fiber, she is assumed inaccessible to the
detection process. By separating the transmission noise and detection noise, we
can always extract a larger key compared with the case that all loss and noises
are induced by an eavesdropper's interference. An asymptotic key rate against
collective Gaussian attacks can be determined numerically for the given
channels' parameters. The improvement of the key rates turns out to be more
significant for the reverse-reconciliation scheme.
