I'm writing a paper and my advisor is requiring that all large numbers have a comma as a thousands separator. I've read this question which asks about how to make siunitx not throw errors when there is a comma in the input, but how can I force a comma in the output when using \SI?
Ideally it would only do it for numbers with more than 4 digits -- ie. \SI{9000} would not contain a space or comma but \SI{10000} would be 10,000. But if that's not possible, I'd settle for 9,000 also.

;-)– egreg Sep 18 '13 at 20:28