I think there are some factors, like your age, that they figure into the quad results. We refused amnio, because my (then tripple screen) came back possitive. When we looked into it, my age would have tipped it possitive no matter what they found.
If you want to go and find out more, then go have another ultrasound or sonagram, but you certainly do not have to. If you refuse, you can count on being hounded about it for the rest of your pregnancy, I went to the hospital with a card that said "refused amniocentisis" that I had to show when I checked in.
Since we were not going to abort the baby if they confirmed downs, we did not think that it was worth the risk, event though they told us that the test somehow justified us taking the small 1-2% risk of miscarrage. You have to ask yourself these questions too, if they do reccomend amnio.
Downs is not the end of the world, but only you can decide if you want to know for sure. If downs is all they suspect, there is nothing you can do to change that condition, so having this test is either about giving you time to get used to it, or giving you time to terminate. For the record, my child did not have downs, so every possitive is not going to lead to heart break.
Good luck.
M.