Why can't i calculate the Inductive reactance in this way?
You are correct that \$Z=4+jX_L\$, and \$Z=\frac{V_1}{I}=\frac{40}{8}=5\$, so \$5=4+jX_L\$. However in \$5=4+jX_L\$ the term \$jX_L\$ is a complex number therefore to solve you must use \$5=\sqrt{4^2+X^2_L}\$,so \$X_L=3\$