Lineup iMac 2015 enlarged with two new members, which is responsible mission - to update the bar for desktop workstations. The choice a potential buyer will be complicated by the fact that the model implicitly compete with each other.

Today you can see complete compare models. If you have iMac 2012 Late or 2014 Mid, you can see how is old your iMac. :)

iMac 21.5-inch 4K Model 2015 models:
The Retina 4K iMac has a 2.15-inch (diagonal) display boasting 4096 x 2304 pixels at 219 ppi. That's slightly more than standard 3840 x 2160 video 4K, which should allow for full-size editing with some interface room to spare.

iMac 21.5-inch 2014, 2013, 2012 Models:
The Standard iMac has a 2.15-inch (diagonal) display boasting 1920 x 1080 pixels at 109 ppi. T's FullHD, which means there are a true 1920 pixels in width and 1080 pixels in height, and the display is not over-scanning, under-scanning, or reinterpreting the signal to a lower resolution.