Class 4 should be fine. I don't think you'll notice the speed difference in this case. My 950 has reached its limit on the main memory, and I am probably going to use an SD card soon too. I used to keep my books on a combination of main memory and SD on my older model 505. I didn't have to restart every time I transferred new books to see them.
Here is an old thread with a similar question regarding acceptable types of memory card:
https://www.mobileread.com/forums/sho...d.php?t=106978