b^8=a^4 => log[a^4]b^8=log[a^4]a^4 => log[a^4]b^8=1 => 8log[a^4]b=1 => log[a^4]b=1/8

Или: log[a^4]b=(1/4)log[a]b=(1/4)1/log[b]a=(1/4)*(1/2)=1/8.