I can confirm that this PSU is working fine so far. I have just played some T-Zer0 followed by some Ruff 'n' Tumble, and I am now posting this message, all from the A4000.
The PSU I am using is badged by "FSP Group Inc" and the model number is "FSP200-50PLA". It's not actually micro ATX, it's a strange long thin power supply called "Flex ATX". You can find a picture of it
here.
It's roughly the same length as the A4000's original PSU housing so it does fit inside. Because it is not very deep there will be additional clearance above the PSU to put cables etc. It is also very quiet compared to the A4000 PSU.
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moto