hilde45 OP303 posts
The 4v issue is weird, because the multimeter does not read 4v on a properly wired outlet on a different circuit.
Apple to oranges, imo.
What is the wire gauge size of the other branch circuit? What is the total length of the branch circuit? What is the total connected load on the circuit? What is the type of devices that are connected to the circuit? How many CFL and or LED lights are connected to the branch circuit? What is the voltage drop on the circuit that you didn’t measured the 4V neutral to ground voltage?
You have all kinds of things going on with the branch circuit where the wall outlets have reversed polarity issues. You now know the branch circuit feeds outlets on the first floor and the basement. You now know it feeds ceiling lighting. How many CFL and or LED lights? They spew all kinds of harmonic noise back on the AC mains. Who knows what that does to your cheap multimeter.
There is still a very good chance the 4V is caused by voltage drop. Now that you know the branch circuit feeds outlets and ceiling lighting on the first floor, not just the basement, turn off all the connected loads that are connected to the branch circuit. Check for the neutral to ground voltage again. No load, no voltage drop.
As for the power conditioner showing an AC mains ’Line Fault’ condition. Correcting the AC polarity reversal problem, will solve the power conditioner 'Line Fault' problem.