Complex sentence: The seventh major revision of the iMac was announced in October 2015, its main feature a "Retina 4K" display at a resolution of 4096 x 2304 pixels.
Split into simpler sentences: The seventh major revision of the iMac was announced in October 2015. Its main feature is a "Retina 4K" display at a resolution of 4096 x 2304 pixels.