How to use a microphone and record video at the same time using iphone 4s?

I want to record some music video but the audio quality is not that great, I decide to buy a microphone to improve the sound. So the question is can i just plug the microphone in and start recording by using the iphone's camera. If not, do I have to sync audio and video together using different devices?

Are you actually shooting a music video? They are typically shot by lip-syncing and then mixing the session audio in later.

Or, are you using your phone to capture something off of your DVR and put it on YouTube? Because if you are, please just don't. Don't ever do that. It's horrid.

If the Mic is suitable for the iphone 4s then you can plug it in. You can also record audio on a separate device and sinc that with your video using an editor of your choice.