I totally see your point here! You were wrong to read her messages, but I can't blame you for it!
After 4 years, to hear that your partner isn't sure whether they love you or not is heart-wrenching!
I imagine you've talked about what she had said and how it's effected you both?
That's the important thing, communication. We can't really say much else to help as we don't know how it's been exactly, but talk about it, even if it scares you that she might leave!