Stepping out of bounds prevents a defender from establishing or maintaining Legal Guarding Position (LGP). LGP only gives a player who is moving the potential to take a charge. LGP is not required for a player who is not moving. If B3 gets to that spot first, he is entitled to keep it without being run over by A4.
edited to add: The screening rule (4-40) says nothing about keeping both feet inbounds.
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