Filament 4 is not a confirmed bone, receiving a quality grade of “C.” It weakly satisfies our criterion 4 and appears to be an interarm filament that lies between the Scutum-Centaurus and Norma-4kpc arms, in p-v. With an aspect ratio of \(40:1\), it also fails our criterion 6. Finally, it only weakly satisfies our criterion 1 (largely continuous mid-infrared extinction feature), as there is a small break in the feature around \(l=21.25^\circ\).