The Strunk and White solution is technically correct, but it's hardly pretty. You'd be better off rearranging the sentence, to something like:
"We had four plates: one square, one oval, and two circular."
or
"We had one square, one oval, and two circular plates."
Plurality vs. singularity of the noun will always depend on the plurality/singularity of the immediately-adjacent adjective. So, in the first example, you could rearrange the three list items and still have "four plates" be correct.
The word "plate(s)" is omitted multiple times to avoid repetition in both of these examples, but you've got to assume that it's still there to see why your original sentence would have been correct anyway:
"We had two circular plates, one square plate, and one oval plate."
You've all been 100% grammatically correct so far; it's just a matter of style.
(Sidenote: I love this thread)
ethrg wrote:Isn't the English language grand!
Quite 