• PZT8 and PZT4 are the common “hard” piezoceramic materials used in power ultrasonic transducers (e.g., welding, cutting, sonar, etc.).
• PZT8 is perceived as the better choice for resonant devices, primarily due to its higher mechanical quality factor Qm. PZT8 is also considered a “harder” material compared to PZT4, since it has better stability at higher preloads and drive levels.
• PZT4 is used in signal transmitting, it has clear advantages such as higher output (i.e., higher d33), mostly applications: sonar, underwater acoustic transmitter.

So in a word, the Piezo Ceramic disks and plates for ultrasonic transducers applications:
Piezoceramics for ultrasonic cleaning PZT-4
Standard dimensions and specifications for SPZT-4 piezo ceramic disks – for ultrasonic cleaning applications
Piezoceramics for ultrasonic welding PZT-8
Standard dimensions and specifications for SPZT-8 piezo ceramic disks – for ultrasonic welding applications
Disk and plate ultrasonic cleaning transducers
Disc and plate transducers are installed at bottom of cleaning tank and operate at frequencies from 35kHz to several MHz. These transducers are ideal for creating a compact and efficient cleaning system, particularly suitable for precision cleaning eg. in semiconductor and optical manufacturing processes.
Poreless Piezo Ceramic Materials
Poreless piezoceramic has much higher specific (ultrasonic) power density, high strength, wide amplitude characteristics, low aging rate, and extremely low porosity in comparison to any other piezoceramics.