But: If you said to a dog, "Down!" wouldn't that be an interjection? That would be six, wouldn't it?

I respectfully disagree. If you said "Down!" to a dog, you would be issuing a command with the implied verb "get." Down is a preposition here (either that or an adverb -- it's too early for me to tell). Unless of course you had just poured the dog a martini and were ordering him to drink it, in which case you would be uttering a command with the verb "to down" in the imperative.