  By means of synchrotron x-ray and electron diffraction, we studied the
structural changes at the charge order transition $T_{CO}$=176 K in the
mixed-valence quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$. Below $T_{CO}$ we
find satellite peaks indicating a commensurate structural modulation with the
same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears
at low temperature, similarly to the case of simple perovskites like
La$_{0.5}$Ca$_{0.5}$MnO$_3$. In the present case, the modulated structure
together with the observation of a large entropy change at $T_{CO}$ gives
evidence of a rare case of full Mn$^{3+}$/Mn$^{4+}$ charge and orbital order
consistent with the Goodenough-Kanamori model.
