Are you pouring your concrete into the bottom of the hole as a footing? Or are you going to pour it around the posts? If the second, are you going to use sono tubes to pour your concrete into? The reason I ask, is because frost needs to get under something to lift it, if you pour concrete directly into a dug hole it takes the shape of that cavity, giving the frost ample opportunities to latch onto all the inconsistencies of the rough surface. So even if the post is 4’ deep the frost could grab onto the rough concrete at 6” and still lift your post. If your assembly has a smooth outer surface the frost can’t lift it unless it can actually get under the bottom of it. All that said, I would not advise the use of concrete to backfill pressure treated posts to begin with. But if you’re going to do it, at least use sono tubes.