partitions >2gb will by deafault be formatted to FAT32 with 4kb cluster size. the trick is to format it to FAT16 (mostly referred to as just FAT) wich uses a deafault cluster size of 32kb thus marking 2GB as the max partition size. However, you can use a 4GB card with FAT16 by explicitly specifing a cluster size of 64kb.
i still dont understand how a slow card could prevent you from playing homebrew?