Can't put an NEC Article on that real quick....but.. Good old common sense (NOT in NEC) seems to say to ME, it's going to deflect, sag, bend, etc. Depending on where you are putting it, and what's going in/out of it.
Honestly, I would support it from the longer dimensions; strut, framing (wood) or something before the narrow dimension.
Now, IF it's in a wall cavity (2x) that may be another case, but I would support on both 4" sides.
314.23 also says that the box is considered supported by the raceway only if there's more than 1 conduit and only if they are made up into threaded hubs on the box. It doesn't apply to a regular sheet metal box.
Sorry it's taken me so long to get back to this. Things have been busy here. I want to hang this box below some duct work in a pedestrian aisle at a point where there is poor visibility of approaching forklift cross traffic. I will then mount a micro-strobe and annunciator on the bottom edge and apply an adhesive label to the front and back of the box that says "Forklift approaching" The stobe and annunciator will be triggered by existing sensors to warn pedestrians of approaching forklifts (who's drivers can't see past their load)