I have the solution guys... Though its hard to explain, its not very hard to accomplish.
I took my n4 for another try in a different lab. The technician from the start suspected it's not something in the charging port itself, so he asked me to leave the phone for him for a few days.
After 4 days of trying and learning, he managed to find that the problem is in the socket on the board itself is causing the problems, along with the charging port, in my case.
He replaced the port on the board itself and the problem is solved.
Eventually, I paid 40€ for this problem, not very high, beats the other option (buying a new phone)
Try it guys.