All installed memory should be the same speed for best performance. This Mac has four available memory slots. Memory; Storage; Upgrade advice. . Backed by. inch (diagonal) Retina 5K display with IPS technology · GHz · 8GB (two 4GB) of MHz DDR3 memory; four SO-DIMM slots, user accessible · About your iMac (Retina 5K, inch, ) iMac20,1. Your computer's memory capability: Slots: 4 (2 banks of 2). Slot numbers for iMac (Retina 5K, inch, ); iMac (Retina 5K, 27 i5 and i7 Quad Core iMac computers come with both top memory slots. Having four identical apple OEM rams but the system won't turn on with all four slots installed. Install in pairs for best performance. Intuitive and packed with powerful tools, you'll be able to take your ideas to the next level, and the 27" display will further elevate your experience. It will with 3, with 2 but not with 4. Maximum memory. Also, it.