Not sure if this is the most efficient way to go about doing things, but you can set up four different storage units, one for each tape drive, and then create a policy for each storage group and point each policy to a different storage unit.
After reading Mouse's suggestion, I guess I was off base as to where you were coming from. His is definately the way to go, but if you want to control which backup goes to which drive, you could do what I suggested.
