Is there any sonic difference between Rega Fono MC, Mk 3 and Mk 4?

I've been planning an upgrade of my MC phono preamp and considering the purchase of a used Rega Fono MC to support my P3/RB300 'table currently equipped with a Denon DL103r. There has been a lot of positive commentary about the Rega Fono MC and there seem to be at least 2 revisions to the original model the Fono MC; namely the Fono MC Mk3 and the Mk4. However I can't find any data on the sonic performance difference between these models.

Also, there doesn't seem to be any model number anywhere on the case of either the Fono MC and the Fono MC Mk3 that clearly differentiates one model from another, and the Mk4 clearly has a different case.

So my questions are these:

How do I tell the difference between these models?

Is there any sonic advantage to be gained by buying the newer MK4 compared to the older models at half the price?

Thanks in advance for your valued input.