I always put up a 5 strand barbed wire fence. The one exception is a stretch of fence I have that runs through a woods, and that one is a four strand. I did it that way because it is in an area used heavily by deer, and I thought they might be able to cross through 4 strands better than 5. So far, I've had no problems with the cattle getting out, and the deer have not loosened the wire up.
Fencing laws vary from state to state. Around here, when someone wants to build a line fence, they usually just build it themselves, and put up what they want, never asking the neighbor to share the costs, although under Kentucky law they would be required to do so.
