Previewed at WWDC, launching in the fall.
Apple Planning to Launch 4K 21.5-Inch iMac in Late October
A new 21.5-inch Retina iMac has been anticipated for several months, following the discovery of code in OS X El Capitan that pointed towards a 21.5-inch machine with a resolution of 4096 x 2304. That code pointed towards Broadwell chips with Intel Iris Pro Graphics 6200 and AMD Radeon M380 - M395X discrete graphics, which may hint at the hardware we'll see in the updated iMac.
Reliable KGI Securities analyst Ming-Chi Kuo also said in early August that the iMac line would receive a refresh this quarter, adding new processors and improved display quality that brings greater color saturation.
Apple began updating its iMac lineup last October, with the launch of the 27-inch Retina iMac. Updates across the line have been slow, however, due to Intel's ongoing Broadwell chip delays, and the 21.5-inch iMac has not seen an update since 2013.
It is not clear if Apple will release multiple 21.5-inch iMac models during the October refresh, or which chips those machines might use. Skylake chips appropriate for the iMac are launching "later this year," but there are a handful of Broadwell chips appropriate for various 21.5-inch iMac models.