From what you have posted I would guess the problem might be the power supply. Simply testing the power supply with a voltmeter is not always enough. The power supply might have a noise problem which would only show up on the oscilloscope. And of course, the good oscilloscope will cost a lot more than a decent power supply. If you are an experienced builder, surely you must have another power supply handy that you can use for testing. Good to have a spare.
As for RAM, Windows 10 should work just fine on 4 GB. In fact, it works okay on one gigabyte, but maybe a little slow. I would suggest try using less RAM, not more.
Do you have another GPU you could use just for testing?
Have you done an exhaustive test on your hard disk drive? Do you have another hard drive you could use just to see?
The fact that you have tried to install Windows 10 four times would indicate that there is something very wrong with your system. Possibly you have a bad copy of Windows 10. Can you use another computer to download a copy of Windows 10 that is known to be reliable? Even an older version of Windows 10, even one that is over a year old, still should work good.
Here's something you can do,
but only as a diagnostic. Get a good copy of one dollar Linux demo DVDs. A recent version of Ubuntu or Linux mint should work fine. But now if Linux has some trouble installing, you probably have a serious hardware problem.
Even when temps appear to be okay you can have hotspots that will cause failures. Especially in RAM. To spot a bad temp in a RAM stick you would have to have a thermal probe to see how hot the RAM gets. Or just use some other RAM sticks
when testing RAM sticks you can take sometimes an hour or more to be billed catch a problem. This is especially true if the RAM is overheating. The RAM has to be perfect, close is not good enough.
Also, testing your hard disk drive should be a vigorous test that tests every sector on the hard drive. This can take quite a while.
I assume you have another computer handy to help you get a fresh download of Windows 10. In your tool chest you should already have an older copy of Windows 10 on a DVD. Like maybe 1609, oldie but a good one.
That's all I can think of.
I hope this gives you some ideas.
End of dictation.