My experience with Fourth Walls is that the current characters post and visiting characters can come in and comment. I've never seen it done the other way, so I can't really comment on the pros/cons of it. Regardless of who posts, the posting content might be the same if currents post or visitors post.
I know I want to bring in a few visitors, and I would feel more inclined (and comfortable) to comment to a current character's post rather than make my own, simply because I'm bringing in a character to tag a specific character/set of characters, if that makes sense? I feel like any visitors coming in would do a basic intro post. And I have to admit, it's a bit intimidating having to make a post when all you want to do is maybe troll a few people. I know my approach to Fourth Wall isn't everyone's, though.
I'd be up for a mingle post, too. That sounds like a great idea - and a little less intimidating, like it was stated in the post - when it comes to having to write up a starting point.