The R4 Revolution, mainly dubbed Revolution 4, is a series of Nintendo DS storage devices

Nintendo DS storage devices

Nintendo DS storage devices are some of the devices used to store a licensed developer's work-in-progress images, homebrew video games, or downloaded commercial games, since the Nintendo DS is not sold with a rewritable storage medium....

 that is widely popular around the world. To date, only two versions are known: the original R4 and the Revolution 4 'Happy Box'. Both storage devices are both not compatible with the Nintendo DSi

Nintendo DSi

The is a handheld game system created by Nintendo and launched in 2008 and 2009 in Japan, North America, PAL territories, and other regions. It is the third iteration of the Nintendo DS, and its primary market rival is Sony's PlayStation Portable...

 and Nintendo DSi XL. However, there has been a new recent R4 made for the DSi and DSi XL.