Re: Next step
You have been dating this girl for over four years! Don't get me wrong, but this is rather unusual these days. Why for so long, if I may ask?
Why does your mother not like her? Do you think this could have an impact on your relationship with this girl? Have you ever tried to show your mother that there is no valid reason for her to dislike the girl, unless the girl has done something bad to your mother, but I don't think this is the case.
Is the girl currently pressing you to marry her? Subtly or not?
It is really difficult to tell you what to do. From your post we get the impression that there are more cons than pros in this relationship. I guess you guys need to talk about the future and be honest about what each of you expect from the other and what each of you is willing to offer the other.
Then make the decision to stay or break up. It won't be easy, but it's better to face this now than down the road into marriage.