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.
My personal opinion, not criticism.