Name of 4-conductor twinned headphone wire
Wow, I thought that would be an easy search. Then it wasn't. Then I was really curious. 

Looks like the cable you are referring to is called Dual-Pair Zip-Cord From wikipedia, zip cord:


  refers to two or more electrical conductors held together by an insulating jacket that can be easily separated simply by pulling apart.


Searching "double-pair" or "dual-pair" seems to bring up the cable you are referring to. Belden comes up a lot as the manufacturer.

Example catalog (ctrl-f "zip"):

http://www.belden.com/products/catalogs/mastercatalog/brilliance/upload/19Broadcast_Cables.pdf

An example of a store listing:

http://www.interstatewire.com/double-pair-cable-1802b-z4b1000.html

Though a lot of the examples I'm seeing are shielded, twisted pairs. I'm not sure where the cheap, old headphone cables are hiding. You may want to invest in a bunch of heat shrink and a good heat gun for your uneven lengths.