I can replicate the issue you are talking about in the messaging app. About an inch of the screen at the top is slightly darker. But only noticeable on certain colors. I tried all sorts of pictures and other things, I can only see it in the messaging all. The issue is there with all screen modes.
The S II had a very similar issue. There was a triangle of screen that was lighter than the rest, and under certain circumstances you could see black splotches. But both issues were only apparent when you worked hard to see it, but never under "normal" use.
From what I understand having a little inconsistency in AMOLED screens is unavoidable. I would rather have this issue than the washed out colors of an LCD screen, personally. I would wager the issue is probably there on every S4 if you look hard enough, it was on every S II I ever saw.
The only issue I saw that I felt was worth getting a new device was burn in. On the Captivate it happened after about 6 months, the S II about 9 months. I'm hoping that Trend (each phone lasting longer than previous gen until it burns in) continues with the Note, S III, Note II, on to the S4.
Sent from my SAMSUNG-SGH-I337
> HTC 8525
> iPhone 3G
> iPhone 3GS (it was a dark time in my life, I was confused)
> Palm Pre
> HTC Aria
> Samsung Captivate
> LG Thrill
> Samsung Focus
> Samsung Galaxy S II (I777)
> HTC One X
> Samsung Galaxy Note II (I317)
> Samsung Galaxy S 4 (I337)
+ HP TouchPad