Screener with Ball

[zippittyay]

I was under the general impression that there are no circumstances where a defensive player can run over a player with the ball, especially one who is not dribbling. Then this happened.

Player A4 has the ball in the high post position and has not yet put the ball on the floor and has shoulders parallel to the basket and has pivoted on his left foot (the one closest to the basket). Player A3 curls around the high side of A4 from his backside and is soon followed by B4. A4 (with the ball)pivots out of the way of A3 but gets run over by B4 who was never seen by A4. The call is made that A4 has set an illegal screen and is given a foul.

Is A4 obligated to keep both feet set with the ball in hand while 2 defenders close in from either side mearly because a team mate passed by?