Upgrading DL4000 to Windows 2012 R2 is an unsupported upgrade. The upgrade path is not a technical issue because it is purely an OEM licensing limitation leaving the DL4000 constricted to Server 2012 OS. The only operating system expected to be in a DL4000 is Server 2012, which could lead to driver problems. If a DL4000 is upgraded to Server 2012 R2 or newer, the hardware and OS will not be supported by Dell ProSupport.
If you have any additional questions about upgrading the DL4000 to Windows 2012 R2 or newer please contact Dell Pro-Support.
To contact the ProSupport team please give them a call at (866) 362-5350.