How do you calculate log_4 43 with a calculator?

$\ln \frac{43}{\ln} \left(4\right)$
If your calculator doesn't have the ${\log}_{4}$ function, you should know that the definition of ${\log}_{n} \left(x\right)$ is:
${\log}_{\textcolor{red}{n}} \left(\textcolor{b l u e}{x}\right) = \ln \frac{\textcolor{b l u e}{x}}{\ln} \left(\textcolor{red}{n}\right)$
note : ${\log}_{10}$ is often written just $\log$