The first is a new 4K version of the 21.5-inch iMac. Apple's current generation 21.5-inch iMac sports an LED-backlit Full HD 1080p (1920x1080) display with Intel HD Graphics 5000, Intel Iris Pro Graphics, or an NVIDIA GeForce GT 750M GPU with 1GB of GDDR5 memory. There's also a 27-inch iMac with a 2560x1440 resolution IPS panel and GeForce GT 755M graphics, and of course the 27-inch iMac with Retina 5K display (5120x2880) and AMD Radeon R9 M290 or M290X graphics.
A 21.5-inch iMac with 4K display doesn't yet exist, nor has Apple announced that one is coming. However, El Capitan's code base suggests it's on the way, as it make references to Mac support for a new 4096x2304 resolution. The 4K panel reference isn't found in OS X Yosemite.
As to the graphics, El Capitan also includes mentions of Intel's Iris Pro 6200 chipset. This would be a decent option to power a 4K display, though users who want more pixel pushing muscle may have the option to upgrade -- there are references to four new AMD Radeon R9 GPUs, including the M380, M390, M395, and M395X.
Moving on, El Capitan reveals that Apple is working on a new Bluetooth remote control. The new hardware integrates a dedicated Bluetooth chip that can connect devices with an infrared sensor. It also includes a multi-touch trackpad with inertial scrolling support, and potentially Force Touch.
Could a new Bluetooth remote mean that Apple is revamping its set-top Apple TV box? That's the likely scenario, though there are no references to such a device in El Capitan.