PLC devices do not have network addresses, and they are not routers. They are equivalent to a network cable, no more no less.

When a PLC has nothing connected to its LAN port, it enters standby mode after a while. What exactly a "while" is depends on make and model.

PLCs do not show up as regular network devices, for they do not occupy an IP address. They're fully transparent to that layer of networking. You need specific PLC software to see them.

Make sure the four of them are actually all on the same powerline network, and also make sure your network uses your very own individual encryption passphrase. This is best done by plugging them all...

I know for a fact, from lots of experience in even the oldest of houses (100 years and up), that MIMO is never an issue, other than for DSL interference if the phone wire runs in the same tubes as...

See, that is exactly the kind of pointless and useless advice you keep getting here. Activating that feature cuts performance in half, for it disables MIMO. It will not resolve the five-units problem.