Apparently, the Samsung Galaxy S4 battery swelling issue is becoming a fairly widespread issue. Customers started complaining about overheating, endurance degradation, and swelling of the battery. The problem has grown to affect nearly 30% of users by one estimate. Samsung has acknowledged the issue and is actually now offering replacements for those affected.

We are aware of this issue, which has affected a limited number of customers. We ask all affected customers to please visit their nearest Samsung Electronics service centre, where they can receive a replacement battery for free of charge. We remain committed to providing the best possible user experience for our customers.
Sound of if you have experienced this problem.

Source: PhoneArena