The class stats are for sustained speed, which don't say much about
how well it serves in a camera when it's not relying on sustained
speeds, which it usuall isn't. Tests show that cards in the same
class aren't necessarily consistant in general responsiveness in
cameras, but it is a guarentee of at least something.
I've seen lots of posts from people with problems with 4 GB non HC SD
cards ofcertain brands in cameras, because they use a kludge in
addressing to get the larger capacity.
This thread has some explanations:
http://tinyurl.com/2vlnuh
Unfortunately, Transcend, which had always been my go to for cheap and
reliable 1 GB SDs, seems to have not hit the mark with compatability
with their non-HC 4 GB fast SD cards, according to user reviews on
price sites, though I'm sure they're not alone.