The class 4 is a "faster" card, but I don't believe it does anything to improve performance of the Moultries. Are yours D40's or D50's? That's what I have and I use the Sandisk 2GB class 2 cards, which you can buy at Wal Mart for about $10 each. I don't know if you need 4 GB cards. I believe the 2 GB cards will hold up to around 1800 pictures. I've never even come close to filling up my 2 GB cards, but I do check my cameras every two to three weeks.