Right now it has 2x 16GB 2RX4 PC4-2133P-RBB-11 in slots A1 and A2 (one for each CPU)

I want to upgrade to at least 64GB so can I just buy another two identical sticks?

Do they have to be exactly the same or if any HP 2133 16 GB kit with part number 726719-B21 compatible? I saw many very simillar sticks with a few different letters on ebay.

And is 4x 16GB optimal configuration? It would be in slots A1B1 and A2B2 right?