XXXHDPI (640 PPI) support added to Android 4.3
This probably means that Android is going to be in 4K TVs (or set-top boxes) and maybe even a 4K Nexus 10.
According to Android Police, Android 4.3 has support for XXXHDPI which now supports 640 PPI. Before, it only had support for 480 PPI.
So why does this lead me to conclude that the Nexus 10 will support a 4K display? It's simple. Android tablets pull from resource groups 1 higher than their current PPI requires. Another way of saying this is that the highest pixel density an Android tablet can have is 1 lower than the Android max; that means an Android tablet can now have XXHDPI displays. By having support for resolutions over 480 PPI, the Nexus 10 (and other android tablets) can utilize up to 480 PPI screens.
For purposes of discussion, 4K (in 16:10 terms) is 3840x2400 (4 times 1920x1200).
If the Nexus 10 is 10.1 inches, then at 480 PPI (XXHDPI), the screen would have the resolution of no more than 4111x2569. Obviously, that's more than 3840x2400.
It also enables support for 4.7 inch 2560x1440 screens in phones. (or maybe 6 inches... cough Note 3 cough)