Pressure-treated 8-ft. 4x4s are typically made from “peeler cores,” which are the byproduct of plywood production. When a log is turned on a lathe to produce plywood veneer, the center that remains, called the peeler core, is used as a post. These cores often contain the pith, or center of the log, which doesn't accept pressure treatment as well. Peeler-core posts are more likely to warp and twist than posts cut farther away from a tree's center.
When shopping for 4x4 posts, look at the ends. If growth rings start near the center of the post and expand outward, it's almost certainly made from a peeler core. If the rings are off center, it's not the log's center. Buy 10-ft. posts instead of peeler-core 8-footers and trim them to fit.