16GB Total memory using 4pc of 4GB DDR3, 1600MHz, ECC, CL11, 1R, X8, 1.5V, Unbuffered, DIMM, 240-pin or 
Lifetime warranty for exchange. BY Computer Memory Solutions
BRAND Computer Memory Solutions
We don't use Generic modules to avoid incompatibility issues. 
Guaranteed to work or you get all of your money back including shipping.
Compatible with
HP/Compaq Workstation Z420

Maximum Memory:

32GB (8x 4GB, 400W PSU Chassis)

64GB (600W PSU Chassis)

Expansions:

8 Socket(s)

 Depending on the memory configuration and processor model, the memory speed may run at 1866MHz, 1600MHz, 1333MHz or 1066MHz. See latest ProLiant QuickSpecs for processor models and supported memory speeds. 

1866MHz memory requires E5-2600 v2 or E5-1600 v2 series processors