First Observation of the Decays (B)over-bar(0) -> D+K-pi(+)pi(-) and B- -> (DK-)-K-0 pi(+)pi(-)

First observations of the Cabibbo-suppressed decays (B) over bar (0) -> D+K-pi(+)pi(-) and B- -> (DK-)-K-0 pi(+)pi(-) are reported using 35 pb(-1) of data collected with the LHCb detector. Their branching fractions are measured with respect to the corresponding Cabibbo-favored decays, from which we obtain B((B) over bar (0) -> D+K-pi(+)pi(-))/B((B) over bar (0) -> D+pi(-)pi(+)pi(-) = (5.9 +/- 1.1 +/- 0.5) x 10(-2) and B(B- -> (DK-)-K-0 pi(+)pi(-))/B(B- -> D-0 pi(-)pi(+)pi(-)) = (0.9 +/- 1.3 +/- 0.9) x 10(-2), where the uncertainties are statistical and systematic, respectively. The B- -> (DK-)-K-0 pi(+)pi(-) decay is particularly interesting, as it can be used in a similar way to B- -> (DK-)-K-0 to measure the Cabibbo-Kobayashi-Maskawa phase gamma.