Aligning a broken equation using phantom

Question

I am trying to break an equation and align the characters under each other. Here is an example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
f(a) &= f(b) + 3 + \
&\phantom{{}=f(b) + } 4 \nonumber
\end{align}
\end{document}

This aligns 4 under the plus sign, however, I want 4 to be aligned under 3. Do I need to add a character to the phantom?

---

The code produces

Answer 1

When doing phantoms one often have to remember to have relations and binaries have something to work on, as in +1 and {}+1 does not have the same spacing.

Here we need a {} before the = and after the +:

\phantom{{}=f(b) + {}} 

Answer 2

I would not use \phantom but rather move anchor & before "3" and "4":

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
f(a) = f(b) + {} & 3  + {} \
                 & 4  \nonumber
\end{align}
\end{document}