My 2011 iMac is black on one side white on the other. This thing is junk. I'm about to take it in to try to get it repaired out of warranty, but I won't pay to fix what is clearly a defective display. I'll post back what happens.
So, the screen has had a problem since it was new, and you just now noticed it?
More likely, a recent change, eh?
Electronics do fail occasionally because of age.
"fails earlier than you expect" after nearly 4 years of use, is not necessarily a factory defect, but simply a failure.
Most likely the backlight has partially failed, which might be a problem with the backlight board, which is not a part of the LCD panel, or an internal cable may need to be reseated (not really a factory defect), or the backlight itself, which would certainly implicate the LCD panel.
Ask Apple, maybe they will help you out...