If I have a string of equations whose equal signs are aligned and one of the equations is too long and needs to span multiple lines, how do I create a second alignment. For instance, in the following example
\begin{align*}
f(x)&=eqn1\\
&=eqn2_part1+eqn2_part2\\
& +eqn2_part3\\
&=eqn3\\
&=eqn4\\
\end{align*}
How do I align the equals signs and the plus signs.