PhotoJoJo is selling this attachable boom mic for iphone 4 and 4s. It takes one triple A battery. Fits and clips into the headphone jack.It’s pretty affordable at $40.
“The mic attaches to the headphone jack and adjusts to hold the back of your phone, even over a case! It is super lightweight, so it won’t throw off the balance of your phone as you record.
Shooting your friend’s show? Use the directional setting to capture all of the sound coming from the stage. When you’re interviewing the band afterward, switch to the super-directional setting for more precisely focused recording. The mic works through the LiveAction app, which also lets you upload your videos instantly.”