The stock tail wire on my 4*40 worked great for months (until the plane was destroyed in a mid-air collision). I used the stock wire on a 4*60 ARF and it didn't last two weeks- but I could tell the ARF wire was very weak from the beginning.

If I built another 4*40 I would go with the stock wire. For a 4*60 ARF I would throw the stock wire away and use a Sullivan setup.