you are correct; it seems most people only know rj11 and rj45, so those are the only two i normally reference.
i don't think i said anywhere that you could extend 3 lines on 4 conductor cable. i did say some make a custom cable using cat5 from an rj45 split to two 4 conductor male modular plugs. i prefer to use two separate jacks for a 4-line phone.
every pbx i've encountered, even new ones, are made to operate on POTS line. i know there are others that can't use it, but a lot of people opt for something that can be used on an older wiring system.
Well, I just looked. I'm surprised. The previous system we used at our company required 4-wire, but the new one we use (Lucent), despite using RJ45 plugs, works on 2-wire. I guess you learn something new every day.