Stamped steel boxes (4S, 5S, Handy Box, etc) w/ KO's are not for "portable" use. That means you can't make quadboxes, splitters, portable GFCI's etc out of them . The main reason for this is that the KO's bend inward and may touch the device screws, leading to problems. I believe their UL listing covers this...maybe somebody can post this?
