The difference between a Class 6 and Class 4 Micro SDHC card is the speed at which data can be written to the card’s flash memory. This specification is known as the speed class rating and represents the minimum data transfer rate for that specific Micro SDHC card. In general, the higher the speed class rating, the higher the data transfer rate for the card.
All Micro SDHC cards are separated into speed classes. As of March 2011, there were four speed classes for Micro SDHC cards: Class 2 (2 MB/s), Class 4 (4 MB/s), Class 6 (6 MB/s) and Class 10 (10 MB/s). As with all flash-based memory, actual SDHC transfer speed varies depending on the application. Taking this into consideration, a Class 6 Micro SDHC card transfers data 2 MB/s faster than a Class 4 card.
When deciding between a Class 6 and Class 4 Micro SDHC memory card, consider the intended application. Class 2 cards are recommended for standard definition video recording and low resolution digital cameras. These applications require the least amount of transfer speed for affective use. Class 4 cards are recommended for HDTV video recording. Class 6 cards are recommended for professional video cameras and DSC consecutive shooting. Class 10 cards meet the specifications for full HD video recording and HD still consecutive shooting. HD video and HD photography require the highest data transfer speeds because the resulting file sizes are larger than traditional SD files.
Speed class specifications were originally established with video recording in mind. Many camcorders, video recorders and other devices require constant and reliable transfer speeds to function properly. This has become more important with the advent of high definition video and high resolution digital cameras. By designating a minimum data transfer speed for each class of card, manufacturers can ensure compatibility with a multitude of devices.
The choice between a Class 6 and Class 4 Micro SDHC card should be determined by the intended use. Most manufacturers will recommend which class to use for their products. This guarantees optimal user experience and performance. If a device designed for Class 6 Micro SDHC cards is using a lower speed class card, the result could be corrupted data or dropouts. Cameras may experience lag between shots while the image data is being written to the card. Digital cameras equipped with consecutive shooting or burst mode should opt for higher speed class cards to avoid such situations.