  By analyzing $(448.1\pm2.9)\times10^6$ $\psi(3686)$ events collected with the
BESIII detector operating at the BEPCII collider, the decays of $\chi_{cJ} \to
4K_S^0$ ($J=0,1,2$) are observed for the first time with statistical
significances of 26.5$\sigma$, 5.9$\sigma$ and 11.4$\sigma$, respectively. The
product branching fractions of $\psi(3686)\to\gamma\chi_{cJ}$,
$\chi_{cJ}\to4K_{S}^{0}$ are presented and the branching fractions of
$\chi_{cJ}\to4K_{S}^{0}$ decays are determined to be $\mathcal{B}_{\chi_{c0}\to
4K_S^0}$=$(5.76\pm0.34\pm0.38)$$\times10^{-4}$, $\mathcal{B}_{\chi_{c1}\to
4K_S^0}$=$(0.35\pm0.09\pm0.03)$$\times10^{-4}$ and $\mathcal{B}_{\chi_{c2}\to
4K_S^0}$=$(1.14\pm0.15\pm0.08)$$\times10^{-4}$, where the first uncertainties
are statistical and the second are systematic, respectively.
