The version differences are driven by hardware changes.
Harmonics have been reduced by a different board layout for the low pass filters.
Spurs reduced through a 90mhz low pass filter addition between the if stages. Best would be do download the information from HF Signals web site.
The other change is to the SSB fcrystal filter. The center frequecncy was changed from 12 mhz to 11.055 mhz to reduce interference from so of the Nano clocks that are close to 12 mhz (the USB converter).
With out the hardware changes, the code would not work, and there is not significant changes in operation other than in the calibration procedure. The new firmware goes into transmit mode to set calibration, so a dummy load or 10 mhz antenna is required.
Above are what I remember, and I may have missed some significant changes, especially to the firmware, as after I go my v5 board running, I quickly installed the kd8cec firmware and Nextion display.
All of the test that I have done so far indicate that the harmonics are solved with the changes. I do no have a good enough spectrum analyzer to verify the spurs, though others have reported significant reductions.