Remember that 30 frames per second is real-time / real motion video, but that is for a single video stream. So if you want to record 4 cameras simultaneous, all in real-time/real motion video, you need 120 frames per second and its full unshared resources.

O.K.  1-3fps - Slow Time Lapse Video
Good:  5-7fps - Time Lapse Video
Better:  10-15fps - Near Full Motion Video 
Best:  30fps - Full Motion Video