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
4GB RAM Memory Upgrade for HP Workstation Z400 PC3-10600E DDR3 1333MHz 2Rx8 ECC Unbuffered UB DIMM Module (MemoryMasters) at Amazon.com
HP Z400 Workstation - Memory | HP® Customer Support
4GB EUDIMM Memory for HP Z400 Workstation DDR3L-1600 - Newegg.com