My favorite of the bunch is FourTrack (V. 1.2, Sonoma Wire Works; $9.99), which provides solid functionality and is the only one of the 4-track apps with pan controls. It also lets you upload tracks to your computer as WAV files (or directly to the company's RiffWorks software for Mac and Windows) using Wi-Fi sync. FourTrack has no internal bounce-to-disk function (none of the 4-track apps I found do), but Sonoma Wire Works is planning one for a future update. I really like the simplicity and responsiveness of FourTrack's controls. It has meters that seem fairly accurate, which is important because the iPhone's built-in mic has no gain control so it's easy to overload it. FourTrack even lets you do manual punch-ins. Right now, it doesn't have a metronome, so you'll have to do a spoken count-off if one is needed, but a click-track feature is planned for an upcoming release.
FEATURES, AT A GLANCE
Multitrack: Use bounce for more than four tracks (eg. eight, 12, 16 tracks or more)
Track length unlimited
Recording quality: 16-bit, 44.1kHz
Calibrated meters accurately monitor record and playback levels
Recording clip light: ensure input levels do not cause distortion
Calibrated faders accurately adjust playback level of each track
Pan control lets you move tracks from left to right
Timeline—seek to anywhere in your song instantly
Slide-to-Record prevents over-writing your tracks
Latency compensation, accurate to within 1 ms
Compressor/limiter automatically fattens sound of output mix
Unlimited song count
Bounce—mix song to track 1 and 2 of a new song to record as many tracks as you want
Duplicate Song lets you make a backup of a song and continue working on it in a different file
Metronome—maintain a consistent tempo by recording along with real drums (not MIDI). Select a tempo by entering a number or by tapping. Choose from seven sounds, including three beats by drummer Jason McGerr on his signature Ludwig kit recorded in his Seattle-based studio, Two Sticks Audio. Other sounds (Block A and B, Hi-Hat and Pop) are samples of live drums and percussion courtesy of Discrete Drums. Time signatures include 3/4, 4/4, 6/8 and X/4 (unaccented beats that can be used to play 5 ,7, 9 and other odd meters).
WiFi sync to any computer with a browser
WiFi Sync to RiffWorks
Find out more information about FourTrack.