Unfortunately, my one and only Con experience had me writing a custom adventure. (Good day, that. Forutnately, the Con was structured such that it was forgiving of me running way over time. The players didn't mind though... I was on fire that day.)
If you have access to it, Dragon 201 had an article "The 10 Commandments of Tournament Writing", which I found extremely useful. The commandments were:
1) Thou shalt bend the rules until thou hast broken them
2) Thou shalt create the characters first
3) Thy setting shall be unusual and interesting
4) Thy plot shall be neither too simple nor too complex
5) Thine enemies shall be worthy
6) Thou shalt have witty, reasonable and balanced encounters
7) Thy scoring shall be fair and balanced
8) Thy second round shall be worthy of its players
9) Thou shalt make it fun for the game masters
10) Thou shalt beg, borrow or kidnap a good editor
Of course, none of that's terribly helpful in this situation.
Three things spring to mind:
1) If you can find it, there was the adventure and other materials created for the most recent Gameday. That was created specifically to be run in a 4-hour slot, so might be suitable. (At that time, there was also a degree of discussion about ENWorld doing a set of their own adventures for the next Gameday. Several people have at least started on writing adventures; sadly, I don't think anyone has yet posted an adventure in playable form. My own effort needs a redraft and some maps added, and I don't have time to do that just now.)
2) Also at the Gameday, I know a lot of people used the 20th level pregens and the Icon dragons (found in the Design & Development columns) to good effect. You might consider looking those out.
3) On Wizards.com, they have a 3.5e update of the classic Tomb of Horrors. Given that the Tomb originated for use in tournament play, perhaps you could use that? I would suggest upping the difficulty level a fair bit, and then issuing an open challenge to groups to bring their optimised characters (of the appropriate ECL) to take on the classic death-trap dungeon.
EyeontheMountain said:
Why not play a piece of an adventure? I have seen lots of scenes in the Adventure paths that would make great one-shots.
I'm afraid I must disagree with this, unless you're willing to do a significant amount of adaptation work. Ideally, a Con adventure should be a very short but complete adventure, with beginning, middle and end. Running just a part of an adventure is likely to feel incomplete and unsatisfying, IMO. It would be a bit like watching "Empire Strikes Back" without ever having seen "Star Wars" and without ever being able to see "Return of the Jedi".