HP does not use the ATX standard for its power supplies, generally. They are individually engineered to best meet the needs of the workstation. That is what you'd expect for a workstation.
Gabornico has given you the assembly and spares part numbers (both for the same power supply), and those are for the higher powered one made by HP for the Z400. Most of the Z400s used the lower powered HP power supply.
There are some articles on the internet you can easily find about how to create an adapter and convert some specific higher wattage consumer power supplies (specifically from Corsair) over to work fine in the Z400. Once you look at the research, price and time involved I bet you'll search out a used HP higher powered one via the part numbers from Gabo. That would be plug and play, and what I'd do......
Look up "Z400 power supply hack" on Google.