I don't know about plug-in units, but as far as REN goes, I think that if your exchange is close, and/or the cables are good, then you may be able to get away with more. The REN=4 is what every line should be able to take.
I know this because I came home on one occasion to discover that we were up to REN=6, after certain 'helpful' individuals in the house had plugged phones into every socket that they could find! Our exchange is fairly close, & cables are only 6 years old max.
If you exceed REN=4 then your phones may not ring out when someone calls you.