The elastic constants C-11, C-13, C-33 and C-44 of Oriented poly-4-methyl-1-pentane (P4M1P) films were measured with Brillouin scattering. The photoelastic constants p(11), p(13), p(31) and p(33) were measured by evaluating the integral intensities of the phonon lines. The correlation of the photoelastic constants pis and p(31) with the degree of stretching was determined by evaluating the relative integral intensities of the Brillouin lines with the phonon propagation vector into and perpendicular to the stress direction. The other photoelastic constants were measured by comparing the angle-dependent relative integral intensities of the transverse and longitudinal Brillouin lines. The elastic constants, as well as birefringence, were surprisingly found to be unaffected by further stretching of the sample above the necking region. The density was constant for all degrees of stretching. The photoelastic constants p(11) and p(33) showed significant variation during deformation. The results have been compared with former measurements of polypropylene (PP) by Cavanaugh and Wang. Because the elastic constants are unchanged during deformation, the modulation of the intensities is due to the variation of the photoelastic constants for this material.