You sound like you have settled into a comfortable situation. But if you don't see yourself married to her at some point, after 4 years of living together and 5 years together total, it is not fair to keep leading her on like that. What will eventually happen is she will get tired of waiting on you, force you to make a decision, and when you say you don't want to marry her, you will look like a jerk to her and all her friends and family.
Don't settle for easy and comfortable. Man up and let her know how you feel.
Good luck.
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