I don’t know if this was the case in beta 2, but in beta 3 the new resolution only appears if you’re using discrete graphics.
I’m not sure if this an Intel 4000 limit or just something Apple did on purpose, but it’s likely not to matter since using an external display automatically uses the discrete GPU… now let’s hope Apple doesn’t scrap discrete GPUs in MacBook Pro’s. Driving the internal retina display and an external 4K display surely wouldn’t be possible with the Intel 5000 series, especially if Apple is planning on utilizing DisplayPort 1.2 to do better than the 30 Hz maximum of HDMI 1.4
As I pointed out in the last post, this is the resolution of 4K movie projectors, but 1080 vertical logical pixels is a little limiting for those of us used to the 1200 and 1440 of 24″ and 27″ LED Cinema Displays. 4096 x 2560 (2x2048x1280) would be the more typical 16:10 aspect ratio, but the screen size would still be less than 27″. At the same logical DPI as the 27″ cinema display it would be only 22″; a high 25″ if the slightly lower 24″ Cinema Display’s logical DPI is used.