It just needs to be an event that affects all of your characters. Generally, this shows itself as an event where all the characters are present, even though they don't know each other. Historically, the event involves all the characters eventually knowing each other by the end of the script. Recent examples of this include Reservoir Dogs and Vantage Point.
All the catalyst has to be is something that affects each character. If they are all professional criminals, it could be the death of a judge that sentenced each of them at one point or another. It could be a common contact each had.
However, one caveat. As you write your story, you could very easily get into a swamp where your characters are ONLY connected by this one event. If this becomes the case, then you are writing four different movies. Your characters should mesh, even in some small way.
Think back to Babel, Syriana, Traffic, or Crash. While each character's story is catalyzed individually (like I said, this can be done as a single catalyst if you want), but those stories are intertwined in a linear story structure.
If it's four stories that never mesh except for this one event, you are writing four different screenplays. If these characters do mesh outside the single event, move forward with it.
