When the exponent of a power is s a sum it can be rewritten according to $a^{m+n}=a^m\cdot a^n$. If one of the terms is $1,$ one of the factors is the base, leading to the exponent decreasing by $1$ from the original power. $$8^4=8^{1+3}=8^1\cdot 8^3=8\cdot 8^3.$$