My recommendations
Incase Snap
http://goincase.com/products/detail/snap-battery-case-ec20031
Since the ip4 already has great battery life the minimal extended battery life this battery case provides wasn't an issue for me. What was a main selling point for me with this case was the fact that it used the "standard" iphone charging cable making it a somewhat seamless battery expansion experience. No extra cables needed. Some people have complained about the aesthetics of this case, which I can understand but at the same time it is only used in specific situations.
On a side note I think there is a promotional discount for dealmac. com customers. I think around 40% off?
Apple Store Edition Mophie Juice Pack Air
In addition to the original mophie juice pack air's outstanding reviews, this Apple store exclusive redesign has made some appreciated changes. Dockability is an issue for most battery cases. With this redesign being able to dock your iphone while using a battery case is no longer an issue. The new design allows you to pull off the bottom half and dock your iphone. Another welcomed design change is the more "open" opening at the top of the battery case. While still providing the same great protection as the original case, this bigger opening at the top of the case allows the use of "studio" headphones which usually have a bigger headphone plug.
The biggest issue I'd say this case has would be the price. It's definitely on the higher priced side of whats available out there.
iwow battery case
http://www.iwowcases.com/iphone-4-backup-battery-charger-with-clip-red-p-3545.html
Although not a brand name I'd suggest this case for two specific reasons. 1 being that it uses that iphone sync cable to charge and two that it has a built in kick stand. The lack of a need for an additional cable to charge the battery case is icing on the cake. The kickstand is sprinkles on top of the icing. It isn't as dockable as the mophie but it does provide a similar amount of battery juice.
My honorable mention for a battery case would be the battery case provided by Tekkeon.
http://www.tekkeon.com/products-mypower-iphone-1280.html
Tekkeon provides a slim battery case which can be docked just as easily as the mophie battery case. The main difference is that Tekkeon provides extra batteries to be purchased and swapped out just in case you need even more juice.