Split the equation into three lines because it is too long

How can I split the following equation that goes beyond the margin?

\begin{align}
f_4(s)=\\
\sum_{i=0}^{\Delta_1-1}{\frac{1}{i!}[(-s)^i  \frac{\delta^i}{\delta(s)^i}(\exp(-s^{\frac{2}{\alpha}} \lambda_1 (\eta_1 P_1)^{\frac{2}{\alpha}}C(\alpha,\psi_1))
(\exp(-s^{\frac{2}{\alpha}} \lambda_2 (\eta_2 P_2)^{\frac{2}{\alpha}}C(\alpha,\psi_2))}
\label{eq17}
\end{align}

• Welcome to TeX.SX! As the code stands, there will be two equation numbers. Is it really what you want? Dec 26 '13 at 20:44
• I suggest a label that's more informative than eq17 since it might not be equation 17 as you revise the document. Dec 26 '13 at 20:56

Using multline you can just use \\ to change line.
\begin{multline}