Please check this official 5500mAh S4 battery thread for further information. The product will be available for pre-order in the following days.
5500mAh Extended Battery for Samsung Galaxy S4
The battery is now out and available for pre-order: [HLI-I9500XL] Buy Mugen Power 5500mAh Extended Battery for Samsung Galaxy S4 with Battery Door
Nexus 4 4500mAh Extended Battery Case is now released!
HTC One 3000+mAh Extended Battery Case is due in June
BlackBerry Z10 3000+mAh Extended Battery Case is now released!
S4 5500mAh and 2750mAh Extended Batteries are now released!