4．已知数列$\{a_{n}\}$是正项等比数列，数列$\{b_{n}\}$满足$b_{n}=\log _{2}a_{n}$．若$a_{2}a_{5}a_{8}=2^{12}$，$b_{1}+b_{2}+b_{3}+\dotsb +b_{9}=($　　$)$