The good news is that there's been a it of confusion; you only need one of the four figurines to get Gustav to get on your ship! He should give you one when he gives you the coordinates to Susan's place.
When you arrive at Susan's, you should be able to access her collections and get a reward for having one figurine. That reward should be what you need to get Gustav to come on your ship with you.
If you get back to Gustav and he's still not ready to get on board, you might need to talk with the Furogawa shopkeeper first to get things back on track.