Kingston 4GB 2400MHz DDR4 Non-ECC PC Memory ValueRam 4GB 2400MHz DIMM KVR24N17S6/4 at Amazon.com
4GB DDR4 2133MHz | Desktop/PC RAM Module | Integral
A-Tech 4GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010, Mid 2011, Late 2011), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com