Is there an app for the iPhone 4 that tells the pitch of a note through an audio input?
Garageband can do this. Open a new (or existing) project and add a new "real" track with no effects. On the bottom edge of the window you´ll see a LCD-like window which usually displays the current time or measure in the track; depending on which version of Garageband you´re using, you can now either use the up-/down-arrows to switch to the tuner (tuning fork symbol) or click on the tuning fork symbol directly to select the tuner. Every sound that is picked up by your Mac´s microphone will now be displayed as note (with off-pitch)