Wow this is a horrible situation to be in and I difficult question but I would like to try and help...
First thing - this is not your fault. You did nothing wrong in this situation, your mother's boyfriend is just an f***ed up a**hole and deserves to go to jail. And you should be the one to make sure he does end up there. Be strong and determined about this!
Why?Because it probably won't stop until you take action. People like this, the longer they get away with things, the worse their actions get. He probably feels really smart for doing this for 4 years.
Also if it doesn't happen to you, it may happen to some other girl too. You have a chance to put a stop to it though.
First step, speak to your mother. I'm gonna be honest it could go one of two ways: Either she will come to her senses and help you get this guy out of your life for good, or she will be too blinded by him or whatever, and not believe you fully (even if she suspects something). I can't say which way it will go because I don't know your mum...
How you say it is tough. It's gonna be difficult, just do whatever it takes to get the message across. Maybe tell her you have something to say, and if it's too difficult to talk about, write it down in a letter for her.
If she is a decent mother, she will be on the phone to the police in less than a second. If she doesn't take action, then you can't trust her because she may be blinded by this relationship she's in. It does happen sometimes, but obviously not everyone is like that..
But anyway, regardless of if she tries to cover it up or whatever, your next step is to speak to the authorities. Get to child services, get to a rape phone-line, get to the police.. It doesn't really matter who you speak to, as long as they are officials. I dunno what country you're in otherwise I could give you some phone numbers, but all you've got to do is search Google to find some.
These people WILL get you out of that situation and make things better. It may be hard, but stick with it and I promise this guy will get what he deserves.