there always a reason why a women after 4 years doesnt believe you love her, maybe you hurt her at some point or lost her trust. talk to her tell her your sorry and even you if you didnt do anything wrong still tell her look babe im sorry for anything iv done to you to cause you pain or hurt. im sorry if i havent been the best i can be for you but please give me a chance to show you how mush i love you and how affectionate i am towards you. hug her give her a kiss buy her a gift.
You can show her that you love her by being obedient to her. It all shows in how you relate to her, the way you respond when she sends you and how much you listen and obey her instructions. You also need to establish a good communication where you can talk without getting on each other's nerves.
Based on what you've said here I gave no idea what's happening. What is clear is if things are this tenuous after being together for 4 years there is something (or something's) very wrong in this relationship. I don't know why she doesn't think you love her. Clearly one or both if you have issues. Either get professional help together or get away from each other. Life us entirely too short to live like this.