I thought this too. I supposed that each ghost must be limited to line-of-sight information, and perhaps knowledge of the maze as well. However, I see no such restrictions:
There are no restrictions regarding the actions of Ms Pac-Man: movement in any direction not blocked by a wall is allowed at all times. For the ghost team, on the other hand, three restrictions apply:
The first follows from the original game specifications and prevents a ghost from turning back on itself. In other words, a ghost may only choose its direction at a junction, choosing any of the paths available except the one the ghost used to approach the junction.
The second restriction is adapted from the original game: occasionally there is a global reversal event when all the ghosts suddenly change direction. In the original game this happens when particular conditions are met (such as a specific number of pills having been consumed). In our implementation a global reversal event can happen on any game tick with a small probability. This adds an element of randomness to each game. We believe the first two restrictions are essential to allow for complex game play to emerge.
Finally, the third restriction is competition-specific: in order to prevent the ghosts from spoiling the game by constantly blocking the power pills (in which case the game would result in a stalemate), each level finishes after 3000 time steps; all remaining pills are awarded to Ms Pac-Man. It is thus in the interest of the ghosts to catch Ms Pac-Man as quickly as possible. For testing, it may be useful to eliminate the time limit. In order to do so, just set the value for limit in Constants to a very high value.
It still seems like four ghosts would be enough to trap a Ms Pac-Man whose location is known at all times.