Having four identical apple OEM rams but the system won't turn on with all four slots installed. 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. Maximum memory. All installed memory should be the same speed 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. . Slot numbers for iMac (Retina 5K, inch, ); iMac (Retina 5K, 27 i5 and i7 Quad Core iMac computers come with both top memory slots. Install in pairs for best performance. Backed by. Your computer's memory capability: Slots: 4 (2 banks of 2). It will with 3, with 2 but not with 4. Also, it. Memory; Storage; Upgrade advice. This Mac has four available memory slots.