Little Elephant and Product

A Little Elephant from the Zoo of Lviv likes lucky numbers very much. Everybody knows that the lucky numbers are positive integers whose decimal representation contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not. Let F4(X) be the number of digits 4 in the decimal representation of X, and F7(X) be the number of digits 7 in the decimal representa