I have seen similar issues with systems that are powered 24/7 that really aren't meant to be. If the capacitors for the on-board regulators are not of high quality then over time they swell up and sometimes even leak their guts.
I have a Windows 7 desktop computer that runs 24/7 as my home DVR. It is running the Free Windows Media Center. I have had to replace the motherboard once already because of bulging capacitors. Then the replacement motherboard began to have issues. Since I now knew the problem was capacitors I replaced the ones I found bulging which fixed the problem.
Going back to my failing Z400 the reason it failed was when it got powered down the capacitors had pressure reduced because of no power. The constant pressure then no pressure is what caused it to open up and thus lose it's value causing incorrect power on the motherboard.
Because of this issue you may need more than a few Z400 spare motherboards. If a time comes when you have to power off your Z400 miners a number of them may not power up again without issues.
I will keep the post updated -- as of today I have 70 of the Z400 workstations running (8 more will be coming online to finish my current mining plans).
My oldest one is from May 2017 -- so far just the one has this type of issue.
But, I will be sure to update if any more issues pop up in the interest of accuracy / objectivity.
So... the plot thickens.
I changed the motherboard out -- seemed okay for ~2 days.
BAM -- same exact 4-beep code.
I was a bit frustrated but noted the hard drive was a 13-year old Maxtor... while no errors specifically showed hard drive... HP documents say ANY bad hardware can cause the 4-beep failure.
Changed hard drive to brand-new unit, re-installed all software, and now it's been on since last Thursday.
So... maybe just that old hard drive the whole time!
This is the 3rd refurb that I have replaced the hard drive on... but this first one that caused this type of issue.
It's been basically a full week now after replacing that hard drive -- the system hasn't shut off or had any errors since.
So, I suppose, the motherboard wasn't even bad at all so I will hang onto it as another spare.