According to MacRumours the OS X Mavericks 10.9.3 beta adds support for 4K displays and devices that have a Retina resolution.
According to the site, the new feature works with both the late 2013 Retina MacBook Pro and the new Mac Pro with Dell’s Ultrasharp 24 Ultra HD monitor, both of which they say they have tested with the new beta.
This could mean that alongside Retina support, these devices could also support 4K, and possibly be able to view 4K content without the use of these displays, something which we hope is true.
It could also mean that the rumours of Apple releasing a 4K version of its Thunderbolt Display are true, something which would make sense instead of them selling displays from other companies, alongside that the Thunderbolt display hasn’t been updated for around two years.
As of yet there’ nothing official from Apple, but we can expect to hear more from Apple about it later this year, possibly at their WWDC event.