Do what I do: sit right next to them anyway, and scratch your balls and sniff your fingers and look at them sideways until they move. People just need a little coaxing to be encouraged to do things the proper way.
ROM: Too much fear in this town to ever speak to a stranger on the bus. People would rather uncomfortably stand for 40 minutes at the back exit while mentally penning their rage letter for when they get home. Even then, they have to post it anonymously. Classic Portland.
Maybe 4th seater isn't trying to "claim" seats, but instead just wants to be close to someone. Like the dude enters the men's room ahead of you and claims the middle of three urinals.
_ X _ X X = NOT ACCEPTABLE