Amolchanov,  To clarify, the limitation you noted in your posting (16 GB quad-rank RDIMMs are not supported.) Does NOT pertain to what you are trying. You are installing 4 4GB dimms for a total of 16GB, you are not using 16GB quad ranked dimms though. So that limitation won't apply to you.  You should be able to install the 4GB dimms as you had previously stated, without issue. Let me know how it goes. 
Hello all! I want to increase the memory on the PowerEdge R420 server, because OS not used all CPU cores...Current configuration: - Two processors: Intel(R) Xeon(R) CPU E5-2407 - Two memory: 2 x 4GB (Slot: A1 and B1)I think to buy two memories 2 x 4GB and install them to slots: A4 and B4.  But the documentations (Page 46) have string "NOTE: 16 GB quad-rank RDIMMs are not supported."? what does mean? : [URL]  Thank you for all recomendations!!!
Memory PowerEdge R420