Problems or my iMac

Very strange, works well on Mac mini 2.3GHz dual-core Intel Core i5 mid-2011 (2 slots only) BUT NOT more than 3 slots on my iMac 27 inch Processor Intel Core i7 quad-core 3.4 GHz mid-2011 ( refurbished).
After installing my 4 modules, my 27-inch iMac mid 2011 starting up normally and showed that the 16 GB have been taken into account, then turns itself off after a few minutes, never to successfully launch after several restarts.
I tested the modules 2-2 and they seem to me to work well. The problem has to come from my mac that works only 3 instead of 4 modules or 12 GB.