We report measurements of the Brillouin frequencies of acoustically driven metastable states of superfluid 4 He at T ∼ 1 K. This is done using a stimulated Brillouin gain spectrometer operating on a confined spatio-temporal domain (∼ 40 µm, 200 ns). Our work questions previous experimental results regarding the metastable states of liquid 4 He and suggests that thermal effects or vortices can play a significant role in the cavitation process. Thanks to our time-resolved measurements, we also find that it is possible to probe metastable superfluid 4 He beyond the commonly defined cavitation threshold.