Don't let that single extra bother you. Wire the extra conductor to ground or let itfloat, either way it will be fine. For what we are doing, the silver and the teflon are the characteristics of most importance. My only concern would be that it has the extruded jacket rather than the more flexible wrapped tape jacket.
Of the 4 conductor with wrapped jacket, the cheapest non bulk I found was 1.37/ft for 100 ft. The stuff in that ad, if it goes for $30-$40 is a very decent deal, especially if you are going to make interconnects or speaker cable. The extruded jacket may be a little stiff for long mic cables but maybe not.