I'm quite sure this question has been asked and answered many times over here at DHC.
The general consensus appears to be that extensive 4:3 display can cause some short-term burn-in. However, as implied, this is not a permanent issue that is overcome by reverting to 16x9 viewing after a short duration.
"Burn In" may not be the correct term here. Rather, short term residual artifacts from 4:3 aspect ratio may be visible.
From my own experience, using Sony Bravia LCD sets, I've never noticed this issue whatsoever after extensive 4:3 ratio viewing. I'm not someone who favours an image stretch...preferring to view in native aspect ratio.
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