OP, I'm really sorry to hear the misery you have been going through last 4 years. Well done for standing up for yourself! The only wrong thing - you waited so long to do it and suffered all that time.
If the number of managers is anything to judge for, your company should be fairly big one and there should be an HR department? Usually decent employers have zero tolerance to bullying policies in place. How is your company doing in this area? Employer has some duties towards employees, e.g., ensuring the safe working environment, also not allowing harassment and discrimination etc. Do you have witnesses of different episodes of that person bullying you? And more importantly, would they speak up if asked about them? If yes, I would seriously consider going to talk to HR about starting a formal grievance procedure. You have talked to your manager about what was going on and she hasn't done anything about it! If the assistant manager is her friend, she could have had a serious sit down and warn her off you. She clearly hasn't done it, so you shouldn't rely too much on your manager finally starting to do her manager's duties.
Starting grievance procedure will mean formal investigation of the episodes you would have described, serious talks with the bully and depending on the investigation's findings - the consequences for the bully.
Your question whether her higher position would have more weight when it comes to your situation. Well, if your employer let's some people to get away with being nasty because they are higher up the ranks than others, it's clearly not an honorable and fair employer and it doesn't deserve any decent people working for it. And let's be honest, last four years have been a complete hell for you, if they are not wanting to sort things out, you shouldn't waste your time with them. Don't forget that getting up in the morning and going to work can (and should) be enjoyable. From what you have said, it's not exactly a place you will remember with a smile on your lips 20 years later.
I think you are in a point where either the situation gets fixed or you should look for a new job. Bullying affects your self esteem, confidence, performance and your career prospects, you have to stop it one way or other before the damage on you is too big. A big hug and lots of strengths and luck.