Look, SD cards are not recommended for playing Wii games. The speed of data transfer isn't very fast and TBH I would recommend using USBs. 4 GB is not a bad size if you want to have one game on it at a time (wbfs) as there are a lot of games that are under 4 GB in size. I used to use my 2 GB USB (lol) and then used an 8 GB USB which was a big upgrade for me
I would suggest either buying an 8 GB USB (cheaper these days) or doing what I did a while ago - buy a HDD (can be a 100GB one or a 1TB, up to you) and using it to store your games on and store other files on (format it to FAT32). HDDs aren't too expensive these days - just save up a little.
I'm sorry if I didn't help you or answer your question properly - it's just that USB is miles ahead these days and eventually you'll probably end up using them anyway (I've heard of a lot of problems arising with SD).