How to power multiple amps with one transformer?
In general, if your load is X, and you have a supply that can provide X, then you are "good."

However, for highly current sinking applications like power amps (low impedance,) there is significant risk of one channel actually affecting another channel when drawing from the same source. For best audio quality, you want to de-couple the power supplies for the four channels, and that means four transformers, or four separate windings on one transformer (and separate filtering caps and separate rectifier bridges.)