Argh, no, don't plug headphones into an amp that doesn't pass its voltage checks!!!! This is how you end up with damaged headphones, and there's no additional information that can be provided by doing this.
Terminals 2 and 4 are wired together but differ by 150V, ergo they cannot be wired together.
The weirdness that you have here would have me looking at the junction of the two black wires on the headphone jack. I bet one isn't captured by solder.
