Class 4 (SD cards, speed and compatibility)

I read up on SD cards from good old Wiki. The class rating is the cards basic speed in Mega bytes per second. Each disk has its class printed on it inside a "C", looks like a copyright symbol but with a number in it. Note there is lots of chat about SD card compatibility with Raspberry Pi. The general advice is it stick to known brands. I don't know if this chatter was mainly about Beta boards, it has died down in more resent posts. There is a compatibility list at http://elinux.org/RPi_VerifiedPeripherals#SD_cards