A class 6 could be much faster than a class 4. Classes tell the minimum speed. A class 4 has a minimum of 4 MB/s and probably maxes out at like 4 or 5 MB/s while s class 6 has a minimum of 6 MB/s and could have a max of 30 MB/s depending on the card.
EDIT: Also take in to account that R and W speeds are sometimes different.
Sum it all up... go with a class 6 if you can.