Can DAMIAO V3 and V4 actuators, such as J4340 V3 and J4340 V4, be used together in the same robot project?

Yes. V3 and V4 actuators can be used together in the same robot project.

V3 and V4 refer to different generations of the integrated driver rather than different mechanical motor versions. The main confirmed difference is encoder resolution: V3 uses a 14-bit encoder, while V4 uses a 16-bit encoder.
When mixing V3 and V4 actuators, make sure the controller/software uses the correct communication settings, firmware-compatible configuration, and parameters for each actuator version.
V3 and V4 can coexist in the same system, but should not automatically be treated as drop-in interchangeable without checking the corresponding configuration and firmware requirements.