Assigning a quadrant of the playfield to each ghost seems wrong; they are supposed to surround and chase Pacman, clustering around him until they are not only all in the same quadrant but close together. Barring wrong turns and lack of dexterity on the player's part, teamwork is the only way to become a threat.
You don't really need a good performance from your ghosts, Pacman needs a fighting chance. The ghosts need to follow naive and suboptimal rules, like the original ones.
- At every crossing, go straight if possible, else take the shortest path towards Pacman.
- At every crossing, choose randomly among the paths that don't lead away from Pacman
- Always the shortest path to Pacman
- When choosing a direction at an intersection, exclude from consideration as if obstructed all corridors sections already containing a ghost (so that ghosts spread apart on alternate routes)
- Follow a predictable patrol route until Pacman comes close (e.g. in the ghost's field of view)
- Shortest path to the closest or second closest intersection to Pacman, to somewhere ahead of Pacman's movement, etc.
Note that the actual difficulty of the game depends very strongly on maze design.