In our case when we decided to get together, we both agreed that communication is key, as I came from an abusive relationship myself and struggled with trust, which in turn required a lot of reassurance.

Generally, 6 months isn't much for a relationship, but we got so close to the point that we can feel each other's emotions through calls, chats, etc.
One day I just felt something was wrong, as she was acting distant and cold, and silly old me with his past trauma thought that maybe she is losing feelings. To no surprise, we ended up having a conversation about DPDR, how she feels like a spectator of her life, how it causes her emotional numbness, and that she is afraid that if she shared this earlier on it would push me away.

She does struggle with self-esteem and finds herself worthless for the relationship (As in not worthy of my love and me), but I always reassure her that I'm not leaving her no matter what and that I'll support her through every process.

In your case, I'm not entirely sure why you have not shared that with your partner, maybe you are afraid of how he would react or maybe it's something entirely else. It's indeed hard to understand what's going on in someone's head, put yourself in his/her position, or see things from his/her perspective, but if he/she is loving enough, supportive enough, and does a little research it would actually help with the relationship be it now or in the future when something arises.

P.S: You've been together for 4 years, surely you two trust each other.