The R4 3DS is a flash cartridge designed for the Nintendo 3DS console that enables users to play ROMs of Nintendo DS games. The cartridge has a slot for a micro SD card, which users can load with ROMs for Nintendo DS games, as well as emulators for other consoles.
The R4 series of flash cartridges were originally created for use on the original Nintendo DS console to play only DS games from ROM files, but they have since evolved. The R4 3DS plays not only DS games, but NES, GBA and more. It can also play music and video loaded onto its micro SD card. Because of lawsuits from Nintendo, they have been banned in multiple countries, however they are still available in North America.