Cards that include the last 4 of the PAN in the signature strip typically also include multi-colored watermark text or images under the numbers, which are printed in black. This acts as a fraud deterrent, since a scammer who melted down the embossed numbers and re-embossed new numbers would also have to change the number without destroying the watermark.
Documentation from card networks about card features (which I have offline at my employer) mentions that merchants should validate the digits when they're checking the signature against the customer's ID. Of course, merchants rarely check signatures, so it's unlikely that this number check is ever actually implemented.
With the advent of chip cards, physical security features are quickly becoming less important, as is evidenced by some newer cards which don't even use embossed numbers, or which feature designs where the number is printed on the back or in other atypical locations.