The wave speed c and the mechanical quality factor Qm are lower for the unassembled PZT4 plate, but little difference is seen in assembled transducer compared to PZT8. The resonant frequency and parasitic modes are dominated by the titanium transducer body, but the Qm is dominated by losses at the joint interfaces, and not from the piezoceramics. The impedance Z is ~20 to 30% lower for PZT4, but the dissipation factor DF is slightly higher; the higher DF can result in more internal heating due to dielectric losses in the piezos at resonance drive. The recommended preload range for PZT4 is 44MPa to 96MPa, but significant depoling (lower k) is seen after 70°C heat-treat at 96MPa, but not at 44MPa. PZT4 may have some advantages at higher preloads with respect to improved Qm, but PZT4 seems less tolerant of higher stress than PZT8 (may limit max amplitude drive of tool). The current gain (CG) and voltage gain (VG) for PZT4 are comparable to PZT8. For PZT4 versus PZT8 at 44MPa, the current gain is lower, but voltage gain is higher. For PZT4 versus PZT8 at 96MPa, the current gain and voltage gain are higher. The higher preload at 96MPa reduced k significantly, which increased CG, and also increased Qm (i.e., less joint losses), which increased VG. The short term current gain stability is worse with PZT4 at both 44MPa and 96MPa preloads. Additional longer term testing is needed, but PZT4 does eventually stabilize, albeit in 2-3X longer time than PZT8 (i.e., 5 days versus ~1 month). The power gain (PG) is about the same for PZT4 at 44MPa, but it is better at 96MPa. PZT4 may have the advantage of less internal heating per given PLL drive amplitude of tool. The thermal IR imaging camera data showed PZT4 at 96MPa ran 7% cooler WKDQ3=7IRUWKHVDPHȝPGULYHDPSOLWXGH at steady state. However, the FEA results did not show an advantage for PZT4 with respect to PG when assuming the same Qm, but the assembled PZT4 transducers did show higher Qm. It should be noted that the higher Qm seen for the PZT4 transducers is still within the normal range for the PZT8 transducers (albeit the upper range), so slight mechanical differences (e.g., flatness) at the joint interfaces cannot be ruled out (Sherman et al., 2009, Stansfield, 1991, Wilson, 1991). Overall PZT4 can be a great drop-in replacement for some applications. The operating frequency and parasitic mode spacing for PZT4 is nearly identical to PZT8 (for titanium body), and the current and voltage gains are comparable too. PZT4 may be best suited for continuous drive ultrasonic power applications where gain stability, maximum drive amplitude is less important, and less heating and lower impedance is more desirable. However, higher preload may be required to see the reduced heating benefits with PZT4, since 96MPa was better than PZT8 at 96MPa, but 44MPa was about the same.