We present high-frequency ultrasonic measurements on the filled skutterudite SmOs4Sb12 under hydrostatic pressure. The results clarify that the 4f electrons in this compound transform from delocalized at ambient pressure to localized at high pressures with a crossover pressure of approximately 0.7 GPa. This drastic change in the 4f electrons under pressure is apparently related to the non-Fermi-liquid state, which appears in an intermediate-pressure range of 0.5-1.5 GPa. The results of our analysis strongly suggest that the ferro-octupolar interaction becomes dominant at high pressure. Moreover, we report the pressure dependence of the ultrasonic dispersion, which is due to rattling, over a wide range of ultrasonic frequencies up to 323 MHz. The drastic change in the ultrasonic dispersions and the frequency-dependent elastic anomaly in the C-11 mode at lower temperatures imply a possible coupling between rattling phonons and 4f electrons.