my guess is  the 4 disk may be harder to  adjust  for  lower HP requirements.. maybe even impossible for protection on  lightweight machinery.    4 disk is a LOT of square inches of  grab!

They aren't there solely for    emergency  slippage,   they should  'give'  a little on  shock loads too.  

KInda boils down to the end use,  and what you are trying to protect.