Cybook t4b Format Specification
The new epub thumbnail files (.epub.thn) are what Bookeen calls t4b files. They are very similar to the older t2b thumbnail files they were using in earlier versions of the Cybook firmware. As the name suggests instead of using 2 bits to represent color values 4 bits are now used. This increases the number of colors from 4 to 16. In addition to the increased color range the t4b files now require a header of “t4bp” without the quotes....