I have very little experience with gaming at cons, but I can offer a little advice about running a game within a limited timeframe.
For starters, you will not want to spend a lot of time on character prologues. It is best if you simply narrate the details of how the characters came together, came to be in the Grimm lands, and came to be in their current predicament. This will allow the players to get right into the game, and improve the odds of them reaching a satisfactory conclusion by the end of their 4 hour timespan.
When writing a scenario, you will want to use something that the players are going to be instantly familiar with. The do not have the time for them to explore all the aspects of the Grimm world, so it is better to focus on just one; and the closer that aspect is to something they are already familiar with, the quicker they will adapt to their situation and be able to contribute meaningfully to the story. Pick something that you feel everyone should be able to recognize. Even if it is slightly different in the Grimm world, the players should be able to recognize that it is different from the story that they know.
For example: The players all find themselves in Neverland. But with Peter Pan gone, and the Lost Boys reduced to squabbling and bickering, Neverland is in desperate need of some leadership. However, if the players think that is going to be them, they are going to have to prove it. To make matters worse, Captain Hook has chosen now to vent some more of his frustration on the Lost Boys. He's been turned away several times before, but the wiley Captain is becoming wise to the booby traps that the children have employed to defend Neverland so far; and this time he thinks he has a way to outsmart them. The Lost Boys probably put too much faith in their traps, after all they have always worked before. Will the players be able to thwart Captain Hook and his pirate crew?
Finally, plan for the story to have a satisfying ending. As the game is likely to be a one-shot, don't leave the characters stranded in the Grimm lands with an uncertain future. Make sure the story includes uncovering a means to return home. Ideally, the session should conclude with the players being offered one final decision, return home, or stay in the Grimm lands (perhaps they decide that leading the Lost Boys is a much better deal than what they had back home). This should leave the players satisfied with the game, and feeling like they were in control of the story (although YOU know better ).
Well, that is about it for my advice right now. You may already know most of this if you have run games at a con before, but hopefully someone can make use of it. If nothing else, I hope the scenario suggestion is something that you can use.
Good luck, and good gaming.