Can't you run a 28" 4K panel at 2560x1440 using a HiDPI mode? I run my 13" rMBP at 1440X900, which still looks pretty good. It actually looks pretty good at 16

Can't you run a 28" 4K panel at 2560x1440 using a HiDPI mode? I run my 13" rMBP at 1440X900, which still looks pretty good. It actually looks pretty good at 1680x1050, except that everything is too small for comfortable work. Sure, it wouldn't be "full retina" resolution, but it would look a lot sharper than a regular 27" 1440p display. The only real constraint could be the video chip's ability to render at 2X that resolution and then downscale.