how to downgrade from iOS 6.1.3 to iOS 5.1.1 in iPhone 4? I have an iPhone 4. It has network lock from Switzerland. I want use Gevey SIM on this phone. Gevey SIM is more compatible for iOS 5.1.1.

how to downgrade from iOS 6.1.3 to iOS 5.1.1 in iPhone 4?
It's not possible. Each time you restore an iOS device's OS, Apple must sign as valid the version you are trying to install, and once a version has been considered as deprecated, which usually is a couple of weeks after a new iOS version has been released, Apple stops providing a valid certification for it.