Well, I upgraded my projector to an Epson 5050 this week. But, not because of 4k (its actually fauxK). The blacks on my old LG were going, and a new bulb didn't help, and there isn't a ton left out there that isn't 4k-ish. I don's have a 4k source and per the below I don't think it makes sense to sit 9 feet from the screen!
That said, the 1080p picture is pretty outstanding out of the box, and even better with some calibration. But man, the thing puts off some heat. Its probably not far off from hair dryer heat production.