  In order to beat any type of photon-number-splitting attack, we propose a
protocol for quantum key distributoin (QKD) using 4 different intensities of
pulses. They are vacuum and coherent states with mean photon number $\mu,\mu'$
and $\mu_s$. $\mu_s$ is around 0.55 and this class of pulses are used as the
main signal states. The other two classes of coherent states ($\mu,\mu'$) are
also used signal states but their counting rates should be studied jointly with
the vacuum. We have shown that, given the typical set-up in practice, the key
rate from the main signal pulses is quite close to the theoretically allowed
maximal rate in the case given the small overall transmittance of $10^{-4}$.
