How often is Ratchet key pair changed in signal protocol?
First, how often is a new DH ratchet key is decided by the implementation.
Alice will trigger a DH ratchet step when she receives a new public key in the header of the message sent by bob (section 4.1 of double ratchet specification[0] goes into the detail of it). In this case, B2's header did not have a new public key, while B3 and B4 do.
Coming to your question, It doesn't matter if A2 had a new key or not. Why? Because this protocol allows for messages to arrive with old keys. Think of the case where Alice's message is not delivered but bob keeps sending message(s).
[0] https://signal.org/docs/specifications/doubleratchet/