Outlet box. 4 in. x 4 in. 2 1/8 in. thick. 4-in. square junction boxes are mainly used for multiple wire connections, using proper connectors, in commercial/industrial applications. Box must be closed with a flat junction box cover. Extensions are used to add extra capacity to the box. Used with proper covers, they can also serve for device installations. Concentric knockouts. Made of galvanized steel.