Since the ratio between successive numbers in a geometric sequence is always the same, use the numbers given in the problem to figure out what that ratio is.

$\frac{6}{4} = 1.5$ so each number in the sequence is 1.5 times the previous number.

The first number is $\frac{4}{1.5}$ or $\frac{4}{(3/2)} = 4*\frac{2}{3} = \frac{8}{3}$

The last number is 6*1.5 = 9

So the sum of all four numbers is: $\frac{8}{3} + 4 + 6 + 9 = 21\frac{2}{3}$

Answer: D