That almost makes sense but if we are going to separate America using the tectonic plates as a reference I think that it makes more sense to have 3 Americas (South, Central and North America). Here's a map (from Wikipedia) for reference.
The Carribean Plate (I assume that's the one you mean) is largely an oceanic plate, making it geologically distinct from the more continental North and South Amercian plates. The other continents have all pretty much moved independently of each other, including the Americas, while land on the Carribean Plate is largely the result of it being 'pinched' between North and South America. Plates are more of a cause of continents than a defining factor, but they can serve to be a good indicator. In this case though, it means that sometime in the geologically near future, North and South America will no longer be connected at all, and likely most of Central Amercia will be underwater. I think most people would define continents to avoid having them disappear (though Antartica will be a different issue should all the ice melt).
Defining continents in terms of tectonic plates doesn't seem to be a viable option but I have a more philosophical question now, what's the benefit of defining continents? I understand about separating countries (there are economic and politic factors involved) but why do we try to separate different parts of land into continents when we don't even have a real definition of "continent"?
I've been taking this from a geologic standpoint because that is where my background is. For that, plates works well. But obviously, on a bigger, more holistic level, 'continent' becomes more vague. I think talking geographically and culturally, it's a historical thing largely. As Europeans developed the concept, they created these separate areas because (1) they represented a significant travel time to reach and (2) they were generally culturally different enough to be seen as distinct. Old (again, European) maps have the center in the Mediterranean, not the Atlantic. It was easy to seperate the world into Europe North of the sea, Africa South, and Asia East. Now, I think a lot of the hold over is for ease of education. It's much easier to teach kids based around readily divisible sections with obvious divisions. Africa is an easier concept to describe than say "Middle East." (Which really isn't any better defined than continent.) Also, helps with teaching geologic history, and cultural history. Other than that though, I too see little value in it, especially in a globalised world.
You are right, I completely forgot about history when I asked the question. In your opinion what number of continents is the most accurate?