Class 4 is good enough for pictures and even viewing HD movies.
it's recommended to get a Class 10, if you are doing recording in HD, because of the huge amount of data to be written to the card. Class 10 has faster write access.
also the difference between a Class X Samsung vs a Class X Sandisk is insignificant. it's like comparing Honda and Toyota running at 100km/h. If both are running exactly at 100km/h, who's faster?


for unknown reasons, people usually like to compare different classes, eg Class 10 vs Ultra & etc.
of cos, faster is usually better but it's also more expensive. so at the end of the day, how deep is your pocket also matters.