At 4 inches deep you need 44 cubic feet (cf), at 6 inches you need 72 cf, not sure how many cubic feet in a bag, so if it were 5 cf you would need 11 bags for 4" and 14+ for 6".
I did some research, Cedarest, the one we used is 2.5 cf, so you would need 22 bags for 4" or 28 for 6".
Then add 2...