16GB KIT (4 x 4GB) for HP-Compaq Z Workstation Series Z200 Z400 (4 DIMM Slots)
CMS 12GB (3X4GB) DDR3 10600 1333MHZ ECC Non Registered DIMM Memory Ram Upgrade Compatible with HP/Compaq® Workstation Z200, Z210, Z400, Z600, Z800 for Server Only - B81 at Amazon.com
Solved: Z400 RAM - Page 2 - HP Support Community - 6148900
Renewed HP Z400 WorkStation Intel Xeon CPU W3520 8Gb RAM 500GB HDD DVI Windows 10 With 30 days Return, 90 Days Exchange
A-Tech 24GB ECC Unbuffered Memory Kit (6 x 4GB) for HP Z400 Workstation - ECC UDIMM DDR3 PC3-10600 1333MHz 240-Pin DIMM 2Rx8 1.5V Dual Rank RAM (AM080899) at Amazon.com
HP Z400 Gaming Computer PC GeForce GTX 1060 256GB SSD 12GB RAM [Z400] - $825.00 : Professional Multi Monitor Workstations, Graphics Card Experts
Solved: Z400 RAM Upgrade from 8GB to 12GB or 16GB - HP Support Community - 6746942
CMS 8GB (2X4GB) DDR3 10600 1333MHZ ECC Non Registered DIMM Memory Ram Upgrade Compatible with HP/Compaq® Workstation Z400 Workstation for Server Only - B80 at Amazon.com
How to install 16GB of memory/ram into an HP Z400 Workstation - YouTube