  The first observation of the decays $\bar{B}^0_{s}\to D_s^+K^-\pi^+\pi^-$ and
$\bar{B}^0\to D_s^+K^-\pi^+\pi^-$ are reported using an integrated luminosity
of 1.0 fb$^{-1}$ recorded by the LHCb experiment. The branching fractions,
normalized with respect to $\bar{B}^0_{s}\to D_s^+\pi^-\pi^+\pi^-$ and
$\bar{B}^0_{s}\to D_s^+K^-\pi^+\pi^-$, respectively, are measured to be
{\br(\bar{B}^0_{s}\to D_s^+K^-\pi^+\pi^-)\over\br(\bar{B}^0_{s}\to
D_s^+\pi^-\pi^+\pi^-)} &= (5.2\pm0.5\pm0.3)\times10^{-2}, {\br(\bar{B}^0\to
D_s^+K^-\pi^+\pi^-)\over\br(\bar{B}^0_{s}\to D_s^+K^-^\pi^+\pi^-)} &=
0.54\pm0.07\pm0.07, where the first uncertainty is statistical and the second
is systematic. The $\bar{B}^0_{s}\to D_s^+K^-\pi^+\pi^-$ decay is of particular
interest as it can be used to measure the weak phase $\gamma$. First
observation of the $\bar{B}^0_s\to D_{s1}(2536)^+\pi^-, D_{s1}^+\to
D_s^+\pi^-\pi^+$ decay is also presented, and its branching fraction relative
to $\bar{B}^0_{s}\to D_s^+\pi^-\pi^+\pi^-$ is found to be {\br(\bar{B}^0_s\to
D_{s1}(2536)^+\pi^-, D_{s1}^+\to D_s^+\pi^-\pi^+)\over\br(\bar{B}^0_{s}\to
D_s^+\pi^-\pi^+\pi^-)} &= (4.0\pm1.0\pm0.4)\times10^{-3}.
