4GB PC10600 DDR3 1333MHz SO-DIMM Memory Ram Compatible with 2011 MacBook Pro, Mid 2010/2011 21.5" & 27" iMac Models, 2011 Mac Mini (OWC1333DDR3S4GB) at Amazon.com
A-Tech 4GB RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
Apple MC309LL/A 21.5 inch iMac Core i5 2.5 GHz 4GB RAM 500GB HDD Mid-2011
Apple iMac Core 2 Duo 2 GHz 4GB RAM 120GB SSD El Capitan | refurbishtech.net