All I know is that they do a 12 week and 20 week scan in my local hospital, and try to sex the baby (or babies!) at the 20 week one.
I don't know if they can tell the sex any earlier than 20 weeks or if it just coincides with the scheduled developmental scan.
Also, it's worth remembering that it can be quite hard to tell the sex from the scan. Our local Health Board stopped telling parents for a few years as there had been some bad press about them getting so many wrong! They've started up again recently and folk are still complaining that they were expecting a boy and got a girl, and vice versa.
Very exciting to see your babies though. I've heard the 4D scans are brilliant.