Apparently, the Wii won’t accept an SD card larger then 2 GB, so you can’t use the newer, larger, 4 GB cards. Or so everyone thought.
What you actually can’t do is have an SD card with more than 2 GB of free space - an entirely different kettle of fish.
Hackzine’s got a post on how you can use a card greater than 2 GB in size. Basically, you chuck a bunch of junk data on the card until it only has like 1.9 GB of free space. Then, as you add data to the card, you delete the junk data.
Tada! 4 GB of save files and downloaded games.