Used by hi-fi buffs to measure the purity of copper cables and interconnects. The N stands for "nine".
A cable described as being made from 4N copper is at least 99.99% pure, since there are four nines in the percentage figure. I don't know if I could tell the difference between a 4N and a 5N cable just by listening, though.
See also Reliability Nines for a similar useage.