If you dont like the current dualsim phones on the market and want to have several numbers aktive in your iphone 4, SocBlue A830 may be an interesting solution.
On the upside:
It is a really slim case for iPhone 4, equipped with 2 sim slots and an additional battery. Connected by Bluetooth you will have 3 active numbers controlled by a single iPhone The 2 sim cards in the SocBluecase are controlled by an app
On the downside:
You need to consider that this solution requires a jailbreak
As iPhone 6 jailbreak is not stable yet, you need to run IOS 5.1.1 jb for the moment.