The iPhone 4 is such a device that legally only Apple can repair or do anything to it. This is a kind of difficulty for those mobile enthusiasts and jailbreakers. With network boundaries, you can only run a specific designated SIM in there if Apple says so. If you really don’t want to unlock the Apple device, then wait for the costly but Dual SIM “cover” for the iPhone 4.
This should only make the iPhone twice as much thick but it shouldn’t be an issue for the people that love to carry 2 devices with different carriers. This is the only option available in the original format but yet again, not everyone buys the iPhone 4 to really use it with dual SIM capabilities.