22v So I can drive 4ohm loads safely as they require less voltage... 25v is good for 8ohm, but the LM4780 allows 4ohm operation, as opposed to the 3886 which is best suited to 8ohm, no?
Indeed they do require less voltage. However what is generally done is that the voltage is dropped, to something like 22V, in order to accomodate loads of 3 or 4 ohms and not strain the chip. The chip itself can take it in terms of nominal voltages and the current demanded, but what tends to happen when the dissipation demanded is marginal is that the chip gets almost locked into running too hot, and even quieter passages don't allow the chip to get out of a regime where the current limits come down and protection cuts in lower. Also, by itself, while warmth is a good thing for the sound, over-temp seems not to be.
But you are paralleling the chips so the load each half sees is effectively double what it would see alone. Half the current is demanded and the dissipation is halved. This means you can raise the voltage above 22V, back towards what you would have if you were driving an 8 ohm load. Personally I would take it to 27V. You are dissipating across the whole double chip exactly what you would in a single 3886 driving a 4 ohm load on each channel, but you have more area, though not quite double the 3886.
The 3886 has no problem driving 4 ohm loads and, as I understand it, the LM4780 is simply two 3886s on a single die. Also, if you go for 27V you can always wipe a volt and a bit off by using two bridges instead of one, and that could constitute an improvement too.