  Using data from the FOCUS (E831) experiment at Fermilab, we present a new
measurement of the branching ratio for the Cabibbo-favored decay mode $D^0 \to
K^-K^+\pi^-\pi^+$. From a sample of $2669 \pm 101$ fully reconstructed $D^0 \to
K^-K^+\pi^-\pi^+$ events, we measure $\Gamma(D^0 \to
K^-K^+\pi^-\pi^+)/\Gamma(D^0 \to K^-\pi^-\pi^+\pi^+) = 0.0295 \pm 0.0011(stat.)
\pm 0.0008(syst.)$.
  A coherent amplitude analysis has been performed to determine the resonant
substructure of this decay mode. This analysis reveals a dominant contribution
from $D^0 \to K_1^+ K-$ modes.
