  The four-state continuous-variable quantum key distribution (CVQKD) protocol
has a long practical secure distance \cite{Leverrier_PRL_2009}, while it has
the difficulty of parameter estimation. We propose an improved four-state
protocol, where the covariance matrix can be estimated from experimental data
without using the linear channel assumption, and thus ensuring its
unconditional security in the asymptotical limit. Our new scheme keeps the
advantages of high reconciliation efficiency and long secure distance of the
four-state protocol, and it can be implemented under current technology.
