It should be 64.
If we let x be the first term, y be the second term, and r be the common ratio (that number which we multiply to x to get y, and also the number we multiply to y to get 4, which is the third term), then we get
We also know that
We now have a system of three equations, and three unknowns.
Solving for r gives us
Substituting this in the first equation for x gives
Which, when solved, gives
Thus, since yr = 4, we can deduce that
Finally, since xr = y, then xr = 16, and