if the guppies are doubling, that means whatever they were yesterday, today is 100% more, so the rate of growth is 100%.
[tex]\qquad \textit{Amount for Exponential Growth} \\\\ A=P(1 + r)^t\qquad \begin{cases} A=\textit{accumulated amount}\\ P=\textit{initial amount}\dotfill &4\\ r=rate\to 100\%\to \frac{100}{100}\dotfill &1\\ t=\textit{elapsed time} \end{cases} \\\\\\ A=4(1 + 1.)^{t}\implies A=4(2)^t~\hfill \stackrel{\textit{after 4 days, t = 4}}{A=4(2)^4}\implies A=64[/tex]
