12mm, anything much less is liable to bend if you hit a brick
If you hit anything stout enough to bend #3(US)/#10(Metric)/⅜"/9.5mm rebar, you'd be better off moving to one side of it than trying to plough through it with #4
Stating the bleeding obvious. The point being that a lighter bar could bend BEFORE you manoeuvre it around the obstruction if you are giving it a good clout,12mm won't.
If you hit anything stout enough to bend #3(US)/#10(Metric)/⅜"/9.5mm rebar, you'd be better off moving to one side of it than trying to plough through it with #4