Eye Fatigue is always an issue, looking at any screen can cause it
Its not a question of IF its how long before fatigue sets in and some screens its a short period, maybe an hour, others you are fine even after a full 8 hour shift. I think there is also user sensitivity factor where some are effected more than others.
I watched the demo running on the Sony 4k Wed at Best Buy and it was amazing as well as very easy on the eyes (not on the wallet). Image was very natural looking compared to many of the other big screens around the room, many of which I could not see watching for any extended period unless they had a LOT of adjustment away from the "dynamic buy me now" setting.
If history tells us anything, the first material for a 4k display will be porn, followed by nature stuff and sports. Pay per view seems REAL likely.