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