What is a class 4 pole? A very good question. The strength of wood poles is determined by the length and diameter. The industry established standard critera to rate the strength. Since trees don't grow perfectly round, circumfrence is used instead of diameter. Class refers to the circumfrence range for a given strength. Class 1 is a fat pole. Class 10 is a very skinny pole. A class 4 pole is about 7" in diameter at the top.