[unclebilly]

What type of livestock is he keeping in?

We build barbed wire fences all the time. The corner posts are 5.25 or 6” and the normal posts are 3.25 or 4.25… every 16.5’.

My fences see far more load than the fence in an urban backyard ever will. In the summer cows are running on them constantly trying to get to the grass on the other side. In the winter, the tensile load of the wire from thermal contraction pulls them incredibly tight. A good fence lasts 40+ years. I just replaced one with old split cedar posts that was about 100 years old.

4” is fine especially with an 8’ spacing. The fence will get torn down for other reasons long before it wears out. Stop overthinking this stuff.