Lane Violation

[BMA]

I just finish my first camp, which was great. After I review my tapes more I will have more question for you guys , but here is the first one.
A1 is at the foul line(with the ball), B3 & A4 are in the wrong spaces,(spots 2&3) just before A1 perpares to shoot they switch places. Who is the violation on A4 for moving or could it be on B3 for discernment(not sure on the spelling). My partner called it on A4, cancel the first shot(which was missed) I was not sure so I said OK lets play. I missed asking the observer about it, but since there is a wealth of knowledge on this borad I'll ask you guys.