If you don't intend to change games out often, class 4 is fine. If you get a very large card, a higher class is nice if you change out games or other media on it often just because you write to it a lot.
Highest class ratings are meant for things like digital cameras which have a high burst data write application (the faster they write, the faster you can take more shots, obviously). This (flash-cart) is a far lower-tech memory need.
If you're just going to throw games/homebrew on it once and that's it, class 4 is perfectly fine. There won't be a noticeable difference in the use of the flashcart. It just takes longer to write as others have said, compared to class 6 or 10 cards. There are very few games with save data that is larger than half a megabyte, so the difference shouldn't be noticeable at all for just playing games.
For what it's worth, I have bought a number of ADATA memory products and they have all been top notch, benchmarked higher than their rating, and none have failed (probably 4-5 memory cards and a 120GB SATA III SSD). But my Class 10 16gb MicroSD is from PNY (it was a little cheaper than ADATA at the time of purchase).
Please remember when you get your memory card to give it a formatting with the SD Association Formatter from www.sdcard.org before putting the flashcart's kernel on it and adding the games. The formatter built into Windows don't treat SD cards any differently from any other mass storage device (which is bad, because it is different by specification).
