There is no need to send more coins to A2. That does not "replenish" anything. Here is how it is done, assuming coins were sent from A1 to A2, but the amount of the transaction fee was too low:
1. Send the entire balance of A2 (including the unconfirmed transaction) to an address in your wallet (A4), and pay a transaction fee that is enough to pay for both the A1→A2 and A2→A4 transactions.
That's what I thought when I saw the OP's explanation. He wanted to give a master class on how to do CPFP and he did it incorrectly, when in fact it is very simple. In my case when I did it, instead of sending the balance of the unconfirmed transaction to another address in my wallet, what I did was to take advantage of the fact that I had to make a transaction to pay for something and make the CPFP to do it.