Tested them with the PSU from a working L3+ and the miners still work!! However, it seems like the PSU's are broken. There is no output at all, 0 volt. The fan does turn on. I have unscrewed one, but it doesn't look like something is damaged or burnt.
Any ideas?
Sounds like you've got some crap PSU's, replace the PSU's and don't load up more than 3 of them on a single 20 amp circuit and you'll be fine, 4 is pushing it and I wouldn't recommend that on 20 amps
Short version is if you're kicking 1 breaker, you need to move the 4th unit to a different plug that you know is on a different breaker (preferably with nothing else)
Even then you really are not supposed to use 20 amps of a 20 amp breaker. You should use about 80% to be safe and not melt the line. So 16 Amp Max per line.