The first observation of the decays $bar{B}^0_{s}to D_s^+K^-pi^+pi^-$ and $bar{B}^0to D_s^+K^-pi^+pi^-$ 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^-)overbr(bar{B}^0_{s}to D_s^+pi^-pi^+pi^-)} &= (5.2pm0.5pm0.3)times10^{-2}, {br(bar{B}^0to D_s^+K^-pi^+pi^-)overbr(bar{B}^0_{s}to D_s^+K^-^pi^+pi^-)} &= 0.54pm0.07pm0.07, 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 $bar{B}^0_sto D_{s1}(2536)^+pi^-, D_{s1}^+to D_s^+pi^-pi^+$ 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_sto D_{s1}(2536)^+pi^-, D_{s1}^+to D_s^+pi^-pi^+)overbr(bar{B}^0_{s}to D_s^+pi^-pi^+pi^-)} &= (4.0pm1.0pm0.4)times10^{-3}.
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