Question about Lining up for a free throw

[bradfordwilkins]

Another thread got me thinking.

Lets say we have A1 shooting.

B1 and B2 line up in their spots. A2 lines up appropriatley. A3 decides to stay back with A4 and A5 rather than lining up. Where does B3 and B4 go? Do they leave an empty space for the offensive player? or slide down?

Thanks