In the end, the Mili Powerspring 4 for the Iphone 4 has arrived. I've got it from play.com. They are quite good, actually and cheaper too. I've got it for £40 and it is the cheapest that I can find.
The Mili Powerspring 4 is actually an external battery which will "save" you from the need to get near a wall charger. After installing the device onto the Iphone 4, I do find that the Mili is quite large but it is quite alright for my palm size. I can take pictures even with it locked on to the Iphone 4 without any problems.
However, I do have one reservation though - It is just too heavy! I wish that it is a little bit lighter than that but I think it is just nice for me because I would not need to worry about charging all the time! I think Apple should have installed a higher capacity battery on the Iphone 4, most probably a 1500mAh battery and not a 1420mAh.
Update 1: With the Mili Powerspring 4 installed, it makes the Iphone 4 weights like a brick! It's heavy but worth it as I have no trouble in getting sufficient juice from it! At this moment, I am still using the juice pre-charged on the Mili Powerspring 4. I have not charged it since! Hurray!