We write $$4^n=a^{n-1}\implies 4\times 4^{n-1}=a^{n-1}\implies 4=\left(\frac a4 \right)^{n-1}\implies \frac a4=\sqrt[{n-1}]4\implies a= 4\times \sqrt[{n-1}]4$$
We write $$4^n=a^{n-1}\implies 4\times 4^{n-1}=a^{n-1}\implies 4=\left(\frac a4 \right)^{n-1}\implies \frac a4=\sqrt[{n-1}]4\implies a= 4\times \sqrt[{n-1}]4$$