peter wrote:
I asked several contractor to bid on replacing a section of wood fence.
Most use the same size fence pressure treated post 4x4, which actually
measured 3.5x3.5 inch.

However, one of them says they use a 4.125 x 4.125 size post. I've never
seen this size before. Is it better than 3.5x3.5 or do you think the
contractor just happened to have a cheaper source for this size post?

Since that size is not a standard lumber size, I suspect he is using
rough cut, i.e., not planed. Nothing wrong with that if he is and as
long as it is pressure treated. The rough surface would be a problem
with painting but might add a bit of achitectual interest. For sure
look at a sample of his posts before you sign a contract.

Harry K