In halo 3 the katana was unlocked when you got all achievements and when I finally finished halo 4’s there wasn’t anything for it. Have they said or speculated on anything unlocking maybe in the DLCs or extra achievements that are added later?
nope.
There is an emblem for unlocking all chievos but this seems to be glitched, just like the LASO emblem…at least for some people.