Wow, you guys are really good!
Well, I hate to spoil a good magic trick, but it was a lucky (but educated) guess. Since you said "string quartet" that narrowed it down to violin, viola, cello, or (much less likely) bass. I was going to guess viola, but my sister plays the cello, so I took a stab. The best odds would have been the violin, of course, except that in the US, there are a lot more female violinists than male (at the professional level, anyway) and I didn't know that your group was all guys.
In any case; if I had guessed violin or viola you would have just said "No, I play the cello" and it would have been no big deal. But since I got it right, it's magic, no?