I can propose an autosplit
environment, but it's much better to split the big thing by hand as in the last example.
The optional argument to autosplit
is the fraction of the display width to fill. The default is 0.9
(that is, 90%).
\documentclass{article}
\usepackage{amsmath}
\newenvironment{autosplit}[2][0.9]
{%
\settowidth{\dimen0}{$#2{}$}%
\begin{tabular}{@{}c@{}}
$\displaystyle#2{}$%
\begin{minipage}[t]{\dimexpr#1\displaywidth-\dimen0}
%\raggedright
$\displaystyle
}
{$\end{minipage}\end{tabular}}
\begin{document}
\begin{equation}
\begin{autosplit}{f(x,y,z,w) =}
(x+y+z+\bar{w}) \* (x+y+\bar{z}+\bar{w}) \* (x+\bar{y}+z+w) \*
(x+\bar{y}+\bar{z}+w) \* (x+\bar{y}+\bar{z}+\bar{w}) \* (\bar{x}+y+z+w) \*
(\bar{x}+y+z+\bar{w}) \* (\bar{x}+y+\bar{z}+w) \* (\bar{x}+\bar{y}+z+\bar{w}) \*
(\bar{x}+\bar{y}+\bar{z}+w)
\end{autosplit}
\end{equation}
\begin{equation}
\begin{autosplit}[0.8]{f(x,y,z,w) =}
(x+y+z+\bar{w}) \* (x+y+\bar{z}+\bar{w}) \* (x+\bar{y}+z+w) \*
(x+\bar{y}+\bar{z}+w) \* (x+\bar{y}+\bar{z}+\bar{w}) \* (\bar{x}+y+z+w) \*
(\bar{x}+y+z+\bar{w}) \* (\bar{x}+y+\bar{z}+w) \* (\bar{x}+\bar{y}+z+\bar{w}) \*
(\bar{x}+\bar{y}+\bar{z}+w)
\end{autosplit}
\end{equation}
\begin{equation}
\begin{split}
f(x,y,z,w)={}&
(x+y+z+\bar{w}) (x+y+\bar{z}+\bar{w}) (x+\bar{y}+z+w) \\
{}\cdot{}&
(x+\bar{y}+\bar{z}+w) (x+\bar{y}+\bar{z}+\bar{w}) (\bar{x}+y+z+w) \\
{}\cdot{}&
(\bar{x}+y+z+\bar{w}) (\bar{x}+y+\bar{z}+w) (\bar{x}+\bar{y}+z+\bar{w}) \\
{}\cdot{}&
(\bar{x}+\bar{y}+\bar{z}+w)
\end{split}
\end{equation}
\end{document}

split
allows splitting across lines, but does not do it: the author has to decide where to set the line breaks.