But it's gets worse than that. Some devices will go much slower when you put a class card that is beyond its capability. Be sure your device notes Class 6 support or you may pay for speed and notice it's not so hot.
What is the difference between a Class 4 and Class 6 card if the capacity is the same, in this case 4GB?
Hoarding photos on your phone?
Those picture are hogging memory and could be slowing down your phone.