Spatiotemporal instability in dispersive self-focusing media is investigated on the basis of a modified nonlinearSchrodinger equation (NLSE) beyond the slowly varying envelope approximation. It is found that, for both normaland anomalous dispersions, space-time focusing may lead to the appearance of new temporal instability regionsfor some range of spatial frequencies. The physical origin of the new instability regions lies in the part of spacetime coupling related to the fourth-order dispersion. Furthermore, space-time focusing shrinks the instabilityregions and slightly reduces the maximum growth rates of the original spatiotemporal instability gain spectraobtained from the standard NLSE.