Same disc in a plastic shell buzzer/alarm that hurts ears with 5V. 4000 Hz is close to a B note 3 octaves above middle B. The disc rattles the shell halves, makes crappy speakers.
Stiff plastic cups with piezo on the bottom are not loud as room speakers but they are as headphones. Held lightly the whole cup vibrates, makes more sound. I have used thin steel cookie-tin lids for plates, got some ring in the sounds.
Maybe with an opposite-phase wired piezo on each side, you'd get twice as loud.
More voltage across the wires corresponds to more disc bend.