  We have investigated the local low-energy excitations in CeRu$_4$Sn$_6$, a
material discussed recently in the framework of strongly correlated Weyl
semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS).
The availability of both $^2$F$_\frac{5}{2}$ and $^2$F$_\frac{7}{2}$
excitations of the Ce $4f^1$ configuration in the spectra allows for the
determination of the crystal-electric field parameters that explain
quantitatively the temperature dependence and anisotropy of the magnetic
susceptibility. The absence of an azimuthal dependence in the spectra indicates
that all crystal-electric field states are close to being rotational symmetric.
We show further that the non-negligible impact of the $\check A_6^0$ parameter
on the ground state of CeRu$_4$Sn$_6$ leads to a reduction of the magnetic
moment due to multiplet intermixing. The RIXS results are consistent with
inelastic neutron scattering (INS) data and are compared to the predictions
from \textsl{ab-initio} based electronic structure calculations.
