Line fault at the outlet -- do I need an electrician?

Hilde45, when you measured the 4 volts while the system was plugged in, were the components in the system turned on, or were they turned off or at least in standby? And as far as you know was anything else that is on the same branch turned on at the time?

If a lot of stuff was turned on at the time it increases the likelihood that the explanations cited by Erik and Jim apply. If not, it increases the likelihood that my hypothesis applies, namely that applying 120 volts to the AC neutral input of the component(s) resulted in excessive AC leakage to ground, mainly via their power transformers. Or, as heaudio123 alluded to, if significant current was being drawn by the components or other things the cause could have been a combination of both factors.

If my hypothesis was the main contributor, though, that issue should go away once the hot and neutral connections are reversed to what they should be.

Best regards,
-- Al

It could also be something as simple as a cracked receptacle causing a leak, I’ve seen some look pretty bad.

I have certainly enjoyed this thread as well. There has been a lot of good information suggested and this really points out the problems that can arise when this old house meets do-it-youselfer. I don't mind do-it-youselfer's as long as they get the doing part right. This also demonstrates the effectiveness of the Panamax unit and the importance of a dedicated circuit.

"@ cissado

Good post except in the case of where the duplex receptacle device may have been used as a junction for the make up of neutral and Hot conductors coming in and going out of the outlet box."

I'm not sure what this means exactly. Maybe because I wasn't clear in my post. I usually am not clear... it's my bad writing skills.
I just meant the first receptacle may have been fed from a light fixture. Also, that the wiring inside the light fixture box could be wrong. 

If it was a diy, and I were to guess which of the two boxes is wired wrong, my guess  would have been the light fixture. 

Personally, I wouldn't take down the light first, either. I would guess the first receptacle, possibly disconnect... and go from there. OP shouldn't even do more than that. Give it a quick try and see if it works. Then of course call a pro. 
I'm just bored. I don't post here this much.
Good luck

@ hilde45

Would you please tell us what make and model your multimeter is? Please!

Does it have an auto scale? Is that what you used when you read/measured the 4 volts AC? Are you sure it was 4 Volts or was it possibly 4mv ( 4 millivolts)

Or did you have to set the meter to a voltage scale above, say 150Vac or something like that?

Also when you measured for the neutral to equipment ground voltage did you allow the meter to settle down to a steady state constant voltage?
If you don’t remember would you please repeat the test.

Jim