I like 4K as much as the next guy. It's fun to get really close to my 4k TV and look at the detail. But I think a lot of people don't understand or take into account visual acuity (your eyes ability to perceive detail at different distances). As an example, you start to loose the ability to discern the difference between 4K and 1080p on a 90" display after you get beyond 6 feet. Once you get to about 11 ft away, you might as well be watching 1080p. I also have a 120" projection screen. I'd love 4k on that because at 12 ft away, I'm in the the 4K sweet spot. But I can't justify a 4k projector at this point. Between that and little 4K content, there isn't a compelling reason for apple to move to 4k yet. http://s3.carltonbale.com/resolution_chart.html
I think a lot of people don't understand or take into account that peoples' visual acuity VARIES SIGNIFICANTLY. I can easy make out the difference between 4K and 1080p, even on a 50" TV, from 6 feet away. It's not even a contest.