The extra 4 pins on the 24-pin connector carry 1x3.3V, 1x5V, 1x12V and 1xGND. By plugging the +4 connector in the ATX12V motherboard connection, you shorted at least one of those three lines to ground and shorted the other two to each other.
If the PSU is bulk-regulated, the line(s) shorted to ground should have prevented voltage on other rails from 'ringing up' to their normal voltages.
Connect things properly and try booting again to see how your luck turned out.
Thanks a million for your hwlp!