@UnWired 

Thanks for responding.  So, there is no web page available to the public describing the component differences between the different versions, or how v1, v2, v3, and v4 are different?  (Like, maybe Broadcom quit producing one chip and a newer chip was substituted?)

Even though the basic functions remain the same, it is sort of an expensive experiment to replace four PA9020P with a newer version to see if they is more stable than the V1 model.

@UnWired The main problem here is that TP-Link just loves to abandon older versions of their product from firmware updates, even if they are technically entirely identical, just with a different packaging design.

Other vendors of PLC devices do release the very latest PLC firmware versions even for their oldest hardware.

@UnWired 

That certainly may be the case.  I was rather hoping that the change in version number represented a change in components of some sort. When I read about PowerLine on the web, it seems there are two main chip suppliers and one does better at resisting noise than the other.  If they would simply publish the chip versions I could read up on differences in the chips.  I could probably get out a screwdriver and magnifying glass, open the case, and try to read the chip descriptions.

At one time, I could swear that I came across a web site that listed the components of networking devices, like routers, switches, etc.