I guess you might be correct, a four-line RJ61 can technically be ordered with only one line wired. I don't know how anyone could convey this to a telephone company business office person, but that's another issue. Maybe the customer simply has a four-line set but only need one line connected initially.
On the same note, though, a 6P4C jack can be wired with one line (RJ11) or two lines (RJ14). If you were to call the phone company and ask for an RJ14 with only one line on it, wouldn't that create information overload for them? I mean, let's face it, it's the same thing as the single-line RJ61X when you think about it.
I'll give you the hardware part though!
