  Using a sample of 10 billion $J/{\psi}$ events collected with the BESIII
detector, the decays $\eta' \to \pi^{+}\pi^{-}\pi^{+}\pi^{-}$, $\eta' \to
\pi^{+}\pi^{-}\pi^{0}\pi^{0}$ and $\eta' \to 4 \pi^{0}$ are studied via the
process $J/{\psi}\to\gamma\eta'$. The branching fractions of $\eta' \to
\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ and $\eta' \to \pi^{+}\pi^{-}\pi^{0}$ $\pi^{0}$
are measured to be $( 8.56 \pm 0.25({\rm stat.}) \pm 0.23({\rm syst.}) ) \times
{10^{ - 5}}$ and $(2.12 \pm 0.12({\rm stat.}) \pm 0.10({\rm syst.})) \times
{10^{ - 4}}$, respectively, which are consistent with previous measurements but
with improved precision. No significant $\eta' \to 4 \pi^{0}$ signal is
observed, and the upper limit on the branching fraction of this decay is
determined to be less than $1.24 \times {10^{-5}}$ at the $90\%$ confidence
level. In addition, an amplitude analysis of $\eta' \to
\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ is performed to extract the doubly virtual
isovector form factor $\alpha$ for the first time. The measured value of
$\alpha=1.22 \pm 0.33({\rm stat.}) \pm 0.04({\rm syst.})$, is in agreement with
the prediction of the VMD model.
