Figuring out the memory used in HP workstations takes some patience but there are patterns. Below is information for your Version 2 Z400 (6 rather than 4 memory slots, and the later boot block date in BIOS):
Do a google search for Z400 Spare Parts. The top result is the HP document you want. Under the memory modules heading you'll see the 1,2, and 4GB sticks listed there with their "Spare Part Number" of 536888-001, 536887-001, and 537755-001 respectively. The problem is that if you search on eBay for those three numbers you'll rarely find them. That relates to the difference between the HP Spare P/N and the HP Assembly P/N for the same exact part. So, here is the decoder ring for these:
1GB = 536888-001 = 500208-562 on sticker
2GB = 536887-001 = 500209-562 on sticker
4GB = 537755-001 = 500210-571 or -071 on sticker
Just checking.... on eBay right now I could find the same 2GB stick used for 23.00 each if searching under one P/N but 2 for 6.00 if searching under the other....
If you want to buy used from eBay look for the numbers on the sticker. It is actually even more complicated than this, and you'll see that HP provides memory from multiple brands. They also "bin" the memory to test that the sticks meet their higher specs and that is why I've liked to stick with HP memory.
A memory stick can be held back in speed by a slower processor..... there are different naming conventions for same speeds. It can get quite confusing.
I personally like to get a set of sticks with the HP sticker on each, and the same brand for each so they are truly matching. A HP engineer who is very helpful in this forum advises that filling all slots with matched memory is one of the most important things you can do to optimize performance. I have mixed the brands but not the sticker numbers when necessary.
Finally, if you really want to dig deep then searching for the brand's code alphanumerics that HP uses in eBay may yield you HP sticks that were not listed by the HP sticker number.