Love triangles are always messy..My biggest question for anyone in your situation is 'if she breaks up with her bf for you, what makes you think she won't do the same to you down the road?' its very cynical but it can happen. If she's already having problems, she's gotta work things out or leave. The other problem is, its a very big chance you're a rebound especially since she's been with him for 4 years..
If you really love her, no matter what the outcome will be. You will always want to wish her well, happy, healthy and pretty forever. Love is selfish and it is also hurtful. Everything needed time to get over with. Don't give up her, try your best. Maybe sometime is "love crush". Love Crush as in the girl treat you good, and u think she is a good person make you love her. Usually such case, you must ask yourself. Your "heart"
Love triangles is never a good thing, e veryone just end up being hurt. You should move on, you will probably find a better person in the future
If she chooses you right now, she might go for another betta guy (who is even more her type) in the future? Or what if she chooses her current bf, when you confronted? What would you do then? Being the 3rd party isn't something easy to do.