  We present measurements of the branching fractions for the singly
Cabibbo-suppressed decays $D^+\to K^{+}K^{-}\pi^{+}\pi^{0}$ and $D_s^{+}\to
K^{+}\pi^{-}\pi^{+}\pi^{0}$, and the doubly Cabibbo-suppressed decay $D^{+}\to
K^{+}\pi^{-}\pi^{+}\pi^{0}$, based on 980 ${\rm fb}^{-1}$ of data recorded by
the Belle experiment at the KEKB $e^{+}e^{-}$ collider. We measure these modes
relative to the Cabibbo-favored modes $D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0}$ and
$D_s^{+}\to K^{+}K^{-}\pi^{+}\pi^{0}$. Our results for the ratios of branching
fractions are $B(D^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})/B(D^{+}\to
K^{-}\pi^{+}\pi^{+}\pi^{0}) = (11.32 \pm 0.13 \pm 0.26)\%$, $B(D^{+}\to
K^{+}\pi^{-}\pi^{+}\pi^{0})/B(D^{+}\to K^{-}\pi^{+}\pi^{+}\pi^{0}) = (1.68 \pm
0.11\pm 0.03)\%$, and $B(D_s^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0})/B(D_s^{+}\to
K^{+}K^{-}\pi^{+}\pi^{0}) = (17.13 \pm 0.62 \pm 0.51)\%$, where the
uncertainties are statistical and systematic, respectively. The second value
corresponds to $(5.83\pm 0.42)\times\tan^4\theta_C$, where $\theta_C$ is the
Cabibbo angle; this value is larger than other measured ratios of branching
fractions for a doubly Cabibbo-suppressed charm decay to a Cabibbo-favored
decay. Multiplying these results by world average values for $B(D^{+}\to
K^{-}\pi^{+}\pi^{+}\pi^{0})$ and $B(D_s^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})$
yields $B(D^{+}\to K^{+}K^{-}\pi^{+}\pi^{0})= (7.08\pm 0.08\pm 0.16\pm
0.20)\times10^{-3}$, $B(D^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0})= (1.05\pm 0.07\pm
0.02\pm 0.03)\times10^{-3}$, and $B(D_s^{+}\to K^{+}\pi^{-}\pi^{+}\pi^{0}) =
(9.44\pm 0.34\pm 0.28\pm 0.32)\times10^{-3}$, where the third uncertainty is
due to the branching fraction of the normalization mode. The first two results
are consistent with, but more precise than, the current world averages. The
last result is the first measurement of this branching fraction.
