2005 Dodge Caravan losing oil

my 2005 dodge grand caravan is losing about 4 qts. of oil between oil changes (every three or four months). there are no visible leaks on the top or bottom of the engine, no stains in the driveway, and it does not smoke at start up, or during driving.
there is no sign of oil on or around the muffler.
any suggestions would be appreciatted.

thanks, scott