Impossible to straighten a curled 4 X 4. The best way is to cut it out and replace it. If it curled above the outside rim joist then it's possible if your planing on covering it with vinly post covers to just set a level againt it, scrip a line and cut off what amounts to a wedge and mover the wedge that's removed to the other side.
When it comes to pressure treated lumber ever piece needs to be inspected in the store, Then let it sit out in the sun for a few days to see what's going to mess up as it drys before it gets installed.
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