Sounds like you really messed it up.
To fix it, find the Softmod any Wii guide that is linked in my signature and go through it to get your system current. Include IOS Update even if you think you don't have to.
FAT32 is fine, even for larger games since if you properly install the game, the game image will split along the 4gig limit line into a new file. WiiFlow and the other loaders will handle it just fine. See the FAT32 guide also linked in my signature for people installing techniques.
Chapter 3 and my signature also has recent if not the latest version of the three big USB loaders. Check them out to make sure you are current there.
Then finally, to get Just Dance 4 to work, you will have to lunch the game using cIOs 250 in the loader's game launch settings. Each loaders has their own method of choosing the number to launch with.