G2.4 Let $d$ and $f$ be positive integers and $a_{1}=0.9$. If $a_{i+1}=a_{i}^{2}$ and $\prod_{i=1}^{4} a_{i}=\frac{3^{d}}{f}$, determine the smallest possible value of $d$.
$$
\begin{array}{l}
0.9 \times 0.9^{2} \times 0.9^{4} \times 0.9^{8}=0.9^{15}=\frac{9^{15}}{10^{15}}=\frac{3^{30}}{10^{15}} \\
d=30
\end{array}
$$

Remark: Change the term "least possible value" to "minimum possible value"