As we all know that most common problem with Iphone4 is the Microphone, even how many times you change the mic or mic flex assembly still the problem didn't resolve. Hope this thread will help us analyze the best solution for this issue. I will post the possible solution but let us try to cement the best solution if there's any.
Mic w/ Docking Flex Assembly 
Docking Flex with board system connector 
Audio Codec 
Audio Power Amplifier 
Any successful stories about this issue please confirm. Thanks.
Best Regards,
Mp3