The most appropriate response regarding how the Federal Trade Commission (FTC) would view Tennis Pro International's decision to market its modified packaging as a new product is:
It probably would not approve of it.
The FTC is concerned with advertising practices and deception in marketing. Simply changing the packaging from three balls to four does not fundamentally change the product itself, and thus marketing it as "new" might be considered misleading or deceptive. The FTC generally requires that for a product to be labeled as "new," there should be a significant change in the product that adds value or alters its use.