latex equation super or subscript size

If you wanted to keep the size consistent, you could declare each fraction to use the display style instead.

\begin{equation}
  x = a_0 + \frac{1}{\displaystyle a_1 
          + \frac{1}{\displaystyle a_2 
          + \frac{1}{\displaystyle a_3 + a_4}}}
\end{equation}