We report measurements of charmless hadronic B-0 decays into the pi(+)pi K--(+)pi(-) final state. The analysis uses a sample of 657 x 10(6) B (B) over bar pairs collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider at the resonance. The decay B-0 -> rho K-0(+)pi(-) is observed for the first time; the significance is 5.0 sigma and the corresponding partial branching fraction for M-K pi is an element of (0.75, 1.20) GeV/c(2) is [2.8 +/- 0.5(stat) +/- 0.5(syst)] x 10(-6). We also obtain the first evidence for B-0 -> f(0)(980)K+pi(-) with 3.5 sigma significance and for B-0 -> pi(+)pi K-*(0) with 4.5 sigma significance. For the two-body decays B-0 -> rho K-0*(0) and B-0 -> f(0)(980)K*(0), the significance are 2.7 sigma and 2.5 sigma, respectively, and the upper limits on the branching fractions are 3.4 x 10(-6) and 2.2 x 10(-6) at 90% confidence level.
