There's a lot to like about the Zoom H4n. For $300 you get a small, compact recorder than can record four tracks simultaneously (using the two onboard mics, and two XLR inputs) and it has good sound quality.
BUT, lately it seems that all I hear are professionals putting it down; not for it's recording quality, or even that you can't adjust gain for the channels independently. It's that the device has a habit of corrupting tracks - i.e. they are unreadable - if it runs out of batteries or otherwise stops recording "unexpectedly."
The latest is a post by Anthony Quintano at Quintano Media who got some corrupted files and says they weren't caused by low batteries; though a respondent suggested that he might be using the wrong SD cards with the device (something owners of the H4n might want to check http://www.zoom.co.jp/english/products/h4n/SDlist.html.)
Philip Bloom doesn't like the H4n, and at a recent workshop recommended the Tascam DR100; because it also has level controls for both channels.
If you already own the H4n, should you ditch it? Probably not, though for those once-in-a-lifetime shoots, or if the camera isn't close enough to get backup audio, you might want to have a second recording device going just in case.
Quintano Media: Zoom H4N corrupt files for no good reason...
Amazon: Zoom H4n Handy Portable Digital Recorder
Amazon: Tascam DR100 Portable Digital Recorder