Are the posts that you are editing personal messages, or are they regular posts in private categories? The reason I am asking is because it was my understanding that TL4 users are not supposed to be able to edit personal messages: What does the "trusted users can edit others" setting do?.
Looking at the code more closely now, it seems that TL4 users can edit other user’s PM posts as long as they have permission to create posts in the PM: https://github.com/discourse/discourse/blob/master/lib/guardian/post_guardian.rb#L117. What TL4 users cannot do in a PM is edit the PM’s title or move the PM to a public category.
One other thing that could be related to the issue is are you a category moderator in the category you are editing the posts in?