HP Z4 G4

In what is hopefully the conclusion of my hunt for a cheap tower server supporting REBAR [1] I have just bought a HP Z4 G4 with W-2125 CPU for $320.

Hardware

One interesting thing is that it has an adaptor from SATA power to 8 pin PCIe power. According to Wikipedia the 8 pin connector provides 150W at 12V [2]. According to Wikipedia SATA power cables include 3 12V pins each of which can deliver 1.5A [3] which is 54W. The system as I received it had a single SATA power plug connected so potentially 150W could be drawn from a connector designed for 54W. The first thing I did was to connect a second SATA power connector on the same cable so I could have connectors designed for a total of 108W supplying potentially 150W (and definitely more than 75W).

I found two versions of the specs for this system, this version seems to match what I bought as it references W-21xx CPUs [4] while this version matches what I would rather have with a W-22xx CPU [5]. The URL naming scheme implies that there are potentially at least a few other variants out there. So much for the “buy name brand and you can buy two systems with the same model and have them work the same” benefit you hope to get. Why don’t they just name them “G4.1”, “G4.2”, etc?

It seems that W-21xx and W-22xx CPUs are incompatible, so the W-2295 scoring 30,804 multithread and 2,634 single thread on passmark that I hoped to get isn’t an option [6].

The system is well designed for space efficiency, both it and the Z640 are 17cm wide but the Z4G4 allows my to close the lid with the Intel Battlemage card installed which doesn’t come close to fitting in a Z640. It has 8 DIMM sockets and with the ready availability of 32G DIMMS that allows 256G of RAM which is the maximum the motherboard supports. That compares well to the Z640 that only has 4 DIMM slots and the Z6G4 which only has 6.

The system supports a maximum RAM speed of DDR4-2666 which is better than the DDR4-2400 of the Z640 but less than the DDR4-2933 of the Z6G4.

The NVMe sockets on the motherboard are a convenient feature. Most systems I run need at most two NVMe devices so this saves a PCIe slot which is important when dealing with GPUs that take 2+ slots. Also for systems that don’t really need NVMe I can use some of the small NVMe devices that I have no other use for. 128G NVMe devices aren’t even worth selling and 256G will be of little use in the near future. So when I move to gen4 Z servers I can use up some of them without wasting slots.

Using the lesser socket LGA2066 in the Z4G4 is a minor annoyance, but for a single socket system 18 cores is probably enough.

The BIOS has an option for single-socket NUMA, which is basically locking cores in a single CPU to specific RAM channels. I enabled it but it did nothing presumably because I only have 2 DIMMs. When I get more DIMMs I’ll do some tests of that and compare it with NUMA on my Z840.

Variants

There are many different variants of the Z4G4 and the only way to recognise them is by the CPU not by any part number or serial number AFAIK. The first difference is between server grade CPUs (the W-2xxx CPUs) and desktop grade CPUs (the i7 and i9 CPUs). The systems with i7 and i9 CPUs don’t support ECC RAM which makes them less reliable, gives smaller limits for RAM

The below table compares the Z640 which is my current desktop PC with the Z4G4, Z6G4, and Z8G4 systems. For the latter 3 I have included multiple options for the parts that differ in different models in the same name series. The Z4G4 I have is an early one which only supports W-21xx CPUs which means a maximum RAM speed of 2666 and the best possible CPU would only be 15% faster than my Z640. I can only use this for ML stuff as it’s the only system I have with REBAR support (which works well).

Conclusion

In my previous blog post I concluded that the next step up for me would be DDR5 systems [10]. But now some of the LGA3647 systems are appealing. The Z8G4 would be a decent upgrade from my current Z840 build server and should be affordable long before any two socket DDR5 system becomes affordable.

The Z4G4 doesn’t have any potential for useful upgrades. But for me it was a good cheap way to house a GPU that had already damaged the motherboard of one good system. If the Z4G4 has a PCIe slot break the way my Z840 did then it wouldn’t bother me a lot. It was annoying to discover how limited this variant of the Z4G4 is after buying it, but at that price I can’t complain.

A Z6G4 could be a nice workstation if I found one at a really low price. The only reason I’d seek one out is if I had a need for a desktop workstation with REBAR support, which seems unlikely.

• [1] https://etbe.coker.com.au/2026/05/04/tower-servers-rebar/
• [2] https://en.wikipedia.org/wiki/PCI_Express#6-_and_8-pin_power_connectors
• [3] https://en.wikipedia.org/wiki/SATA#SATA_power_connectors
• [4] https://h20195.www2.hp.com/v2/getpdf.aspx/c05527757.pdf?ver=4
• [5] https://h20195.www2.hp.com/v2/getpdf.aspx/c05527757.pdf
• [6] https://tinyurl.com/2avfb8qe
• [7] https://tinyurl.com/2ddf7t5y
• [8] https://tinyurl.com/kgmagfs
• [9] https://etbe.coker.com.au/2026/04/10/hp-z640-e5-2696-v4/
• [10] https://etbe.coker.com.au/2025/08/02/server-cpu-sockets/