The 10.9.3 beta of OS X contains Retina scaling at 60 Hz.
I’m hopeful this means Retina iMacs this year, as I’m up for a new one since my 2011 27″ is about ready to go on ebay. Not because it’s slow, because it isn’t, but because I’m running out of disk space, and I hate external drives – besides my backup.
In 2011, like always, I got the best BTO iMac they had at that time, and it’s still extremely fast, but a 256GB SSD and a 2TB HDD are just not enough for me anymore.
I guess the 27″ iMac is already very close to Retina, the way Apple defined it. I myself, due to less than 20/20 sight, can’t discern pixels at my usual working distance right now, but it will not hurt to have 4K, especially as I’m very much into photography.