Another slack chain question

No it doesn't. You could add 4 new links to a new chain and it's now longer without any stretching. "stretch" infers some kind of deformation (elastic or otherwise). In this case chains do not stretch they wear.

http://en.wikipedia.org/wiki/Bicycle_chain#Wear
http://www.sheldonbrown.com/chains.html

Exactly, stretch is plastic deformation.