Recently, Yamamoto et al. have carried out torsional oscillator measurements for 4He filled in a nanoporous glass containing pores 2.5 nm in diameter (Gelsil), and reported a large suppression of superfluidity at high pressures. In order to study the mechanism of the superfluid suppression, we have carried out ultrasonic measurements for 4He filled in the same nanoporous glass. We have clearly observed an increase in the sound velocity due to decoupling of the superfluid component. At zero pressure, the superfluid transition temperature TC is suppressed to 1.4 K from the bulk lambda point, 2.17 K. The pressure dependence of TC and the temperature dependence of the superfluid fraction are very different from those reported by Yamamoto et al.