If the "piezo" is rated for 400W, and it can actually draw between 300-500W, then at least a 500W supply is needed, and any supply (power rating) larger will be fine and should not cause significant extra power usage. Of course, the bigger supplies are more expensive.

You might be able to replace one big supply with four smaller ones, but consider if one doesn't startup for some reason - what's going to happen?

And since there are capacitors on their outputs, and you're essentially paralleling four sets of output capacitors, is that going to violate any "maximum output capacitance" requirements?

For something this large (1.5kW) I'd be tempted to consider some type of orchestration/control/power good circuitry, which only "turned on" the piezos once every supply was known working and stable.

Maybe look for a datasheet or app note regarding paralleling such supplies to learn of any caveats.

Could be, that the control loops interfere with each other and the whole lot sings loudly - tread carefully.

Also consider, that if the big supply has an inrush current rating of say, 50A, and the small supplies have an inrush of 20A, then 4x20A = 80A. Might need to sequence their turn-on to prevent a fuse-pop.

These are the reasons why bigger supplies are much more expensive. :)