Also, remember that the nominal voltage for a lipo (3.7V per cell) is when it's almost outa gas. You should really make your choice based on the fully charged voltage. A 4S pack is 16.8 fully charged, so a good choice for this application, a 6S pack would be 25.2V! Spin baby spin!!! Also, since that's a geared starter, you may even be good on 3S.
As John said, the lower the voltage that's able to do the job, the longer your starter will last.
Cheers,
Pete