We report Sb-121.123 nuclear quadrupole resonance (NQR) measurements in the filled-skutterudite superconductor PrOs4Sb12 in the temperature range of 0.05-30 K. The electric field gradients (EFG), V-zz and V-xx - V-yy, at the Sb site exhibit unusual temperature dependence below 30 K. To explain these features, we discuss the coupling between the Sb nuclear quadrupole moment and Pr 4f(2)-derived multipole moments. The observed T dependence of EFG is well explained by the CEF pseudo-quartet consisting of Gamma(1) singlet and Gamma((2))(4) triplet states in the cubic point group T-h. These results, in turn, are indicative of the importance of the coupling between the Sb-121,Sb-123 quadrupole moments and the hexadecapole moment caused by the pseudo-quartet.