According to Zoom, there is apparently a bug in the mp3 encoding software currently used for the DSP unit in the H4. This is sometimes common in newly released products.
 
The following bit rates appear to be affected: 256, 64, 56 and 48.
The following bit rates are error free to the best our knowledge: VBR, 320, 224, 192, 160, 144, 128,
112, 96 and 80.

Zoom expects a firmware update to correct this issue will be available by November 1, 2006. This fix will be available as a downloadable software fix from the Zoom site. Check Samson/Zoom's website for updates. samsontech.com