For an ultrathin NiO(001) film of 4 monolayer (ML) thickness grown on Ag(001), the vibrational properties have been determined by high-resolution electron energy loss spectroscopy (HREELS). For the well-ordered pseudomorphically grown film, nine phonon modes have been identified and their dispersions have been revealed along the (Gamma) over bar(X) over bar high-symmetry direction. The comparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO(001) phonon properties are already fully developed at 4 ML Significant differences are found for the surface-localized phonon S-6 which has an increased dispersion for the ultrathin film. The dipole-active Fuchs-Kliewer phonon-polariton exhibits a narrower lineshape than the mode found for a single-crystal surface, which might hint to a reduced antiferromagnetic coupling in the ultrathin film.