Is this circuit for Piezo Transducer sufficient?
The spec should be read that if you apply a 4 kHz square wave at 3 Vp-p, the transducer will draw less than 5 mA.  The transducer will take up to 25 V, so no worries about overdriving it.

The load on your processor will look like a capacitor rather than a resistor, somewhere around 12 nF.  So if you want to limit the current, it will depend on the operating frequency you choose.

Good luck!