It is considered normal for an engine to use a little oil but not four quarts. How long are you going between oil changes? Also, are you checking the level with the engine off or running?

You might consider adding a small bottle of dye to the oil. When there is a sudden loss of oil, check with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Don't forget to check at the tail pipe.