  Using an $e^+e^-$ annihilation data sample corresponding to an integrated
luminosity of $2.93\,\rm fb^{-1}$ collected at a center-of-mass energy of 3.773
GeV with the BESIII detector, the doubly Cabibbo-suppressed decay $D^+\to
K^+\pi^+\pi^-\pi^0$ is studied with a semileptonic tag method. After removing
the decays containing narrow intermediate resonances, $D^+\to K^+\eta$, $D^+\to
K^+\omega$, and $D^+\to K^+\phi$, the branching fraction for the decay $D^+\to
K^+\pi^+\pi^-\pi^0$ is determined to be $(1.03 \pm 0.12_{\rm stat} \pm
0.06_{\rm syst})\times 10^{-3}$. The ratio of the branching fraction for
$D^+\to K^+\pi^+\pi^-\pi^0$ to its Cabibbo-favored counterpart $D^+\to
K^-\pi^+\pi^+\pi^0$ is measured to be $ (1.65\pm0.21)\%$, corresponding to
$(5.73\pm0.73)\tan^4\theta_C$, where $\theta_C$ is the Cabibbo mixing angle.
These results are consistent with our previous measurement with hadronic tags
but are significantly larger than other doubly Cabibbo-suppressed decays in the
charm sector.
