As far as I know, the Scott catalog has only designated #4387 for the sheet version of the above stamp, however, in a recent purchase I made from the USPS Stamp Fulfillment Services, I noticed the 28c Polar Bear stamp is now being provided in a "reprint" format. I bought a plate block from SFS (right) and compared it to a used copy of the 28c Polar Bear stamp I had in my stockbook (left), which were both scanned together:

The two distinct differences I see are:
(1) The shades of blue around the left eye and into the upper body of the polar bear; and
(2) The shades of purple on the nose, eyes, and left ear on the right stamp (reprint) as compared to the same areas shown in blue on the left (original) printing.
Although these are noticeable differences, I don't know that they are significant enough that the Scott catalog will provide a separate listing for them.
Still, it makes for yet another variety to collect.