Thanks @lancegatlin. This has been a helpful. If using the bullet character to identify the anchor button is easier for users to recognize, I'm going to adopt it in the next Twidor release (2.6) as well. Below is a screenshot with that change. Note that this is not the default view, but rather the expanded view when the user selects "View | Show chords".
Regarding 4-button chords, I think what I'm hearing is although there may be more compact representations, the layout above is far more easily recognizable. So it may be an important element for documentation and certain user interface usability purposes.