Hello, Class 4 is achieved as follows: Write rate: 4.15 MB / sec when reading very quickly (15,4MB / sec) - determined through H2testw V1.4 That's enough but easy to play music, even if you want to play back WAV files in CD quality. Optical class is the shape - really "micro", the little lamp is orange and not very bright, very comfortable, for example when the stick is on the car radio. The form has a small disadvantage: In my Onkyo components (CD players, and receivers) are the USB ports relatively tight, which can at the low and smooth surface of the sticks but be somewhat tricky so often the part must not purely yes and out. Otherwise usual good SanDisk Quality. Preferable to any NoName-product ... but he would have little bit more quickly may be :-)
update: bought ok, second stick with 16GB, performance on the same computer slightly worse: Write rate: 3.88 MB / sec, reading 14,6MB / sec) - determined through H2testw V1.4 So series dispersion present but small.