According to DMTF, DDR4 is type 0x1A, decimal 26.
http://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.0.0.pdf
But you need :
a) A machine with SMBIOS version 3.0 to report it.
b) That Microsoft updates the Windows WMI class with that information.
I don't have a DDR4 machine yet to test it, my latest and newest machines are still on SMBIOS version 2.8, so I cannot help there...
What I can suggest is that you do a report with the mainboard model or machine model that you know for sure are DDR4. Unless you have a very heterogeneous shop (I have, so I can relate to that), it should not be very hard.