Was this sexual abuse?

I’m not sure where to post this but I read through this group a lot and it’s where I feel most “safe” posting.

A little background on me: I’m 20, I live with my boyfriend of 6 years.

I’m not sure how to explain this experience. When I was 4 years old I engaged in sexual activities on multiple different occasions with another girl who who was 9 at the time. I don’t feel comfortable explaining what happened but it was more than just kissing. I know 4 is so young but I know it happened. I can remember it very clearly. The girl was my moms boyfriends sister. I never told anyone. My moms relationship with her bf was short and I haven’t seen any of them since I was very young. Well that girl just died two days ago.. and it just brought up all these memories. I’m trying to be a better me and improve myself. This is my biggest secret. For awhile I was so embarrassed. And then I was angry at that girl for doing those things to me and exposing me to it. And then I hated myself and thought I was gross. And then I felt bad for her and wondered if maybe she had being sexually abused. I wouldn’t say that I was sexually assaulted or sexually abused. I don’t really know what this was. But I know it is my biggest “demon”. I’m not sure what to do because it was so long ago and I was so young. We were both young. And at 4 years old I didn’t know what I was doing I just knew what felt good to my body. 9 years old is also so young. Idk what I’m looking for. I just wanted to put this in writing. I wanted to tell this experience I had and have it actually live somewhere instead of just my head.

Thank you in advance to everyone who reads and replies. Like I said this is not something I’ve never said out loud. Seeing it in writing makes me a little sick.