your calculator probably only evaluates in base 10 (ln is base e but we won't use that today)
so
[tex]log_a(b)=\frac{log_c(b)}{log_c(a)}[/tex]
where c is the new base
so if you wanted in base 10
[tex]log_4(20)=\frac{log_{10}(20)}{log_{10}(4)}[/tex]
using calculator, that is about 2.16096
