We have built a concrete base around a 4 by 4 holding up driveway roof. It measures 25 by 27 and the height is 18 inches. It has 400 pounds of concrete mixture in it. The forms are of thick plywood.
When can we remove them.
You can easily remove them about 12 hours after pouring the concrete. The concrete will still be "green" or make it easier to remove the forms. Hopefully you didn't embed the 4x4 into the concrete, if you did it will lead to the early failure of it because water will not be able to evaporate from within the wood.