Gee, guys, I may have not said it so it was understandable, but if you have 4 wires coming into your house, you can have 4 phone lines. What the phone company does at the outside box is use the same wire pair combination one of you was talking about--and I can't remember which way it is for sure either but I think it is red/yellow for the third line and green/black for the fourth.
With modular jacks, you don't have to hardwire to the new location. Plug in a dual outlet at the current jack, plug in wire with the modular plug to one side, run the wire to the new location, plug the modular plug into the new wall jack, plug the phone into the new wall jack. We have 6 phones on the same line--all done this way.
The teen line sounds great--until one of your kids falls madly in love with someone who lives in the next zone and you get $2-300 phone bills every month. It costs $40 a month for that teen line. And if I had a dollar for every time I have heard "Sure, Dad, I'll pay you back out of my next paycheck." I probably would have broken even. :o) Henry