What you see above is not an ordinary Galaxy S4 case. It has built-in high capacity 3,200 mAh battery that adds up to the default battery of S4 to give nearly twice the talk time without adding a lot of weight and size to the phone.
This Thanko Galaxy S4 case weighs just 90g and comes in both black and white colors. It connects the case using a microUSB cable through which you can charge the phone as well as sync it. When charging the phone through the case, the default battery of the S4 gets charged first and then the 3,200 mAh battery of the case is charged.
For those interested, the Thanko Galaxy S4 case with built-in battery is available for about 3,280 YEN or roughly 33 USD only.