Preview at WWDC likely in June, followed by September launch.
OS X El Capitan Code Hints at 4K 21.5-inch Retina iMac
There's also a mention of Intel Iris Pro Graphics 6200, the graphics chipset that accompanies Intel's newest line of Broadwell processors. Intel Iris Pro Graphics 6200 and a Broadwell/Skylake processor could potentially be destined for a 4K 21.5-inch Retina iMac, but the chipset is also suitable for a number of notebooks. There's a further mention of AMD Radeon M380 - M395X graphics, which could be used in high-end iMac models.
Apple has updated its 27-inch iMac with a 5K Retina resolution, but the 21.5-inch iMac has thus far remained untouched as Apple has had to wait on appropriate chips for the machine. Apple's plans for the 21.5-inch iMac remain unclear, as Intel has only released two desktop-class Broadwell chips and does not plan to release additional desktop chips. For that reason, a 21.5-inch Retina iMac update could still be months off should Apple opt to wait for Broadwell's successor, Skylake.
In addition to pointing towards a potential 21.5-inch iMac upgrade, code in the latest El Capitan beta also hints at a revamped Remote with a multi-touch trackpad that supports scrolling, a dedicated Bluetooth chip, and audio support, potentially for Siri commands. Details on the remote, which will likely accompany the upcoming Apple TV, were first shared in May.