Notice that each new term is half the previous one. For example, 32 is half 64, 16 is half 32, and so on.
Having said that, the constant ratio is 1/2. Using this ratio we can find the next three terms.
[tex]\begin{gathered} 8\cdot\frac{1}{2}=4 \\ 4\cdot\frac{1}{2}=2 \\ 2\cdot\frac{1}{2}=1 \end{gathered}[/tex]