I believe that that the difference is the data transfer and access rate. The Attache 4 is fast than the 3. I just don't know by how much. I have the 4 and I am very pleased with it. It plays back all media files quickly and smoothly. Access to files through your computer programs is also very fast. Almost as fast as the access to the computers own Hard Drive. lastly, data transfer is fast as well both recording and transmitting.