The fourth occupant seat is indeed the second jump seat and is situated behind the copilot's seat.On my company 330 fleet the third occupant seat is situated behind the centre console and is a more substantial design than the fourth occupant seat.
The pilots seats are handed ,i.e they have the substantial arm rest on their outboard and the seat controls inboard.If an occupant was discovered still attached to a seat it should be fairly straight forward to see who was sitting where