The frame rate can be calculated from the number of samples and the duration:
Code:
u_int32_t mp4_numSamples = MP4GetTrackNumberOfSamples(mp4File, trackId);
MP4Duration duration = MP4GetTrackDuration(mp4File, trackId);
double msDuration = (double)(MP4ConvertFromTrackDuration(mp4File, trackId, duration, MP4_MSECS_TIME_SCALE));
double mp4_Frame_Rate = mp4_numSamples * 1000LL / msDuration;
It may also be specified in the video ES.