The engine only has a changeable capacity of around 3 to 3.5 L. If you are loosing that much oil, it should be very visible to where it is going.
It is either
1.. leaking to the bilge. (This amount of oil should be easily visible in the bilge.)
2.. Burning it internally. (Bad rings or valve seals. Either way you would often see blue smoke, which may be hard to see in the water. Try running it on land on the hose)
3.. Burning it through the intake either from the Supercharger seal or the TOPS valve. (Both of these would heavily coat the intake plumbing. The TOPS would coat just below the actual intake filter)
4.. Loosing it to the coolant system. (This would be visible in the coolant tank)
Like mentioned above, the TOPS is a likely candidate.