It is written that "the centers can move towards the 5 circles surrounding the basket" because it is there that the ball will fall. But this is a fight among the centers to get the ball, so if the defenders can block the access to the attackers and be the ones closer to the basket it's fine, it is in the spirit of the game and the sport.
And this was what I thought!
However, in your example you don't say where 5R is because, if he is on the left side of the basket, by staying away to block 4R Blue may leave the rebound very probably for him...
Obviously 5R is in... B1!!
Thank you Miguel.