New 21.5-inch Retina 4K models expected.
'Air Display' to Let You Use the New iPad as a HiDPI 'Retina' Display for Your Mac
Air Display allows you to use your iPad (or iPhone/iPod Touch) as an extra display for your Mac or Windows computer. This old video shows how the app works.
The iPad acts as a wireless second screen that can be used by OS X in the same manner as any other monitor.
The developers were able to get their screen-extending app working with the new iPad with good performance. But using a 2048x1536 iPad as a secondary screen computer screen would result in very small windows, text and user interface elements. So, the team has also included support for Lion's new HiDPI mode. We've covered HiDPI mode in the past. HiDPI mode allows OS X to support high resolution monitors while retaining the same relative size for user interface elements, such as windows and buttons.
HiDPI mode will let the new iPad act like a 1024x768 point screen where each point is made up of 4 pixels. Apple has already built in support for HiDPI and some of their Apps, such as Messages, already include Retina-sized artwork that work in this mode. Users simply have to choose "1024x768 (HiDPI)" mode in their System Preferences to take advantage of the new mode.
Air Display is a $9.99 Universal App in the App Store. [iTunes] The Retina-enabled version will be included in a forthcoming update.