SimpleSplit3DS is a tool for splitting 4GB and 8GB .3ds or .3dz files into .3d# files so you can transfer them to a FAT32 formatted microSD card. It can also join .3d# files back into their original file. It runs on Windows, Mac, and Linux.
Learn more about SimpleSplit3DS by visiting its GitHub page here.
Grab the latest compiled version from the repository's release page here.