Harden steel bench block. Even though it's listed as a 4 x 4 inch it measures just under 4 x 4 but is 3/4 inches thick.
These things are nice and heavy, really good for pounding on.
This bench block is packed in a plastic bag with greasy oil to protect it during shipping and storage. All you need do is wipe it off with a paper towel and you're ready to go.