I got my iPad4 last week and stumbled on this thread.I must be one of the fortunate ones as I don't have this problem on mine.The display is perfect and I am borderline ocd.
I had similar display issues on my Samsung Galaxy Note,only it had black pixelation/crush during dark scenes and not varying contrast.There was a huge thread about that on XDA forums and a lot of people suffered the same issue but a lot of people also claimed theirs was perfect.The cause of the problem was never resolved but the general consensus was that it was hardware.Not 100% of the displays are manufactured identical and from the same plant.
I had similar display issues on my Samsung Galaxy Note,only it had black pixelation/crush during dark scenes and not varying contrast.There was a huge thread about that on XDA forums and a lot of people suffered the same issue but a lot of people also claimed theirs was perfect.The cause of the problem was never resolved but the general consensus was that it was hardware.Not 100% of the displays are manufactured identical and from the same plant.