  The rare decay $\eta' \to \pi^+ \pi^- e^+ e^-$ is studied using a sample of
$1.3 \times 10^9$ $J/\psi$ events collected with the BESIII detector at BEPCII
in 2009 and 2012. The branching fraction is measured with improved precision to
be $\left( 2.42\pm0.05_{stat.}\pm0.08_{syst.} \right) \times 10^{-3}$. Due to
the inclusion of new data, this result supersedes the last BESIII result on
this branching fraction. In addition, the CP-violating asymmetry in the angle
between the decay planes of the $\pi^+\pi^-$-pair and the $e^+e^-$-pair is
investigated. A measurable value would indicate physics beyond the standard
model; the result is $\mathcal{A}_{CP} = \left(2.9\pm3.7_{stat.}\pm1.1_{syst.}
\right)\%$, which is consistent with the standard model expectation of no
CP-violation. The precision is comparable to the asymmetry measurement in the
$K^0_L\to\pi^+\pi^-e^+e^-$ decay where the observed $(14\pm2)\%$ effect is
driven by a standard model mechanism.
