Both yourself and The French's suggested anchor bolts.

Correct me if I'm wrong, but I thought these were like metal rawlplugs in
that they expand in the holes. At only 4inches wide, I would not expect the
concrete to have enough torsional strength to hold them in place. In short,
I would expect to tighten the bolts and have the anchor bolts split the
concrete into two chunks.

If anchor bolts are something else, please fill me in on how they work.

Thanks,
Paul DS.