Well, it is full wave rectified, but that doesn't much matter.
SInce you ought to see about 4.5v of DC< I suspect a plain old standard value 5v relay would work. I have never seen a 4v relay, does someone actually make them?
I don;t know which optos Yamaha used, but if you are building your own circuit, use whatever optos you want. They are running off the same 4.5v supply, so adjust the external current limit resistor to suit.