Sometimes regular four string pickups are used, other times, five string pickups are available.
As long as all the strings are within the sensing area, it doesn't matter if you use a four string pickup.
Fender does pickups with one coil having four pole pieces for two strings, and the other coil having six pole pieces for three strings:
Then there are pickups like Sadowsky's with two three-string sized coils, one of them having a blank spot with no pole pieces.
Considering that the pickups on that BC Rich have no visible pole pieces, I'm guessing that they probably just went with blade style pole pieces that will capture the string's vibration regardless of the string spacing.
Those are probably regular four string pickups.