A memory card is a portable flash chip that retains data even when the power source is turned off. Removable, re-recordable memory cards are used in digital camcorders and cameras, cell phones, personal digital assistants (PDAs), global positioning devices, gaming consoles, digital multimedia players, mobile computers and other personal electronics. Approximately the size of a postage stamp with roughly the same thickness as a credit card, memory cards are available in many capacities that conform to various specifications and standards. A 4G memory card indicates a four Gigabyte (GB) data capacity.
The most common type of flash memory card is Secure Digital (SD), which includes SD High Capacity (SDHC) and SD eXtreme Capacity (SDXC) cards. The Secure Digital Association (SDA) is responsible for setting specifications and standards for these cards. SD also includes the ultra-small miniSD and microSD formats.
MultiMediaCard (MMC) is another type of memory card, slightly thinner than the SD card. It can be used in any SD reader that has drivers for MMC. The open MMC standard includes a variety of alternate, smaller formats. Although SD cards have become dominant, MMCs are still in wide use and a 4G memory card could refer to an MMC card.
Olympus and Fujifilm created the xD-Picture card; yet another type of memory card used in some digital cameras as of 2002. The footprint of this card is different from the SD card, so products made to use the xD card cannot substitute other cards. A 4G memory card is also available in this flavor, though as of 2010, Olympus and Fujifilm are phasing out the xD in favor of manufacturing products that support SD cards.
SD version 1.x (or "standard" SD) originally supported capacities up to 512MB. A revision allowed for capacities up to 4GB. Older products might not be compliant with the revised edition of version 1.x, in which case a 4G memory card will be read as having a lesser capacity. A firmware upgrade might be available for these legacy products.
The succeeding SD 2.0 version (SDHC) picks up where version 1.x left off, at the 4GB threshold. SDHC cards boast capacities from 4GB to 32GB, but SDHC introduced another significant improvement over standard SD cards: SDHC cards are faster.
Unlike standard SD cards, the SDHC specification calls for guaranteed minimum sustained speeds. SDHC cards are designated to be either Class 2, Class, 4 or Class 6 cards, etc., each having a correlating minimum data transfer rate (DTR). The minimum DTR for Class 2 is two Megabytes per second (MB/ps); Class 4 is four MB/ps; and Class 6 is six MB/ps. The maximum DTR is much higher.
This differs from standard (version 1.x) SD cards, which have no minimum DTR. Standard SD cards start from nil speed, building to the maximum write speed, then backing down towards zero as the write process completes. This results in an average DTR that is far lower than the maximum rated speed of the card. In short, a standard SD 4G memory card will be much slower than its SDHC equivalent.
Products that support memory cards are rated for their ability to write data to storage. A product rated at a maximum write speed of 13x is equal to a Class 2 SDHC card’s minimum write speed (MRS). A product rated at 26x is equal to the Class 4 MRS, while Class 6 MRS equals a 40x rating. A speedier card will not make the device run faster.
Before purchasing a 4G memory card, be sure it is supported by your device. SDHC readers (ports/slots) are backward compatible, able to read standard SD cards; but products that only support standard SD will not be able to read SDHC cards. In some cases, firmware upgrades might be available from manufacturers.