You forgot "umami".
It's not so much forgetting it, Iceman, as identifying it.
Umami is not a distinct flavor characteristic, in the sense that salt, sweet, bitter, and sour are. That's what makes it so confusing.
When I was coming up, we said there were five "flavors:" sweet, sour, salt, bitter, and savory. Umami is the Japanese word for what we called savory, and it's a characteristic of certain foods that includes bringing out the full flavor of the other tastes, providing a sort of mellow mouthfeel, and, in general, making food taste better. Some foods just naturally do this better than others---mushrooms, soy, fish sauce, many traditional condiments, etc---and are said to have umami.
The ability of making foods taste better, taste more like themselves, is found in many places. Plain salt is the classic case. If you add salt to any dish (assuming amounts less than it takes to make it taste salty) and compare it to the same dish without salt, you immediately taste a difference. MSG is often touted for this purpose as well.
Umami is merely that idea carried to the ultimate.