You can redefine \@maketitle to give the desired formatting in case of four authors:
\documentclass[preprint,10pt]{sigplanconf}
\usepackage{amsmath}
\makeatletter
\def\@maketitle{%
\begin{center}
\@settitlebanner
\let \thanks = \titlenote
{\leftskip = 0pt plus 0.25\linewidth
\rightskip = 0pt plus 0.25 \linewidth
\parfillskip = 0pt
\spaceskip = .7em
\noindent \LARGE \bfseries \@titletext \par}
\vskip 6pt
\noindent \Large \@subtitletext \par
\vskip 12pt
\ifcase \@authorcount
\@latex@error{No authors were specified for this paper}{}\or
\@titleauthors{i}{}{}\or
\@titleauthors{i}{ii}{}\or
\@titleauthors{i}{ii}{iii}\or
\@titleauthors{i}{ii}{}\@titleauthors{iii}{iv}{}\or% HERE
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{}{}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{viii}{}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{viii}{ix}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{viii}{ix}\@titleauthors{x}{}{}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{viii}{ix}\@titleauthors{x}{xi}{}\or
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{v}{vi}%
\@titleauthors{vii}{viii}{ix}\@titleauthors{x}{xi}{xii}%
\else
\@latex@error{Cannot handle more than 12 authors}{}%
\fi
\vspace{1.75pc}
\end{center}}
\makeatother
\begin{document}
\title{My Title}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\maketitle
\begin{abstract}
\end{abstract}
\end{document}
The change was made in the line marked % HERE which originally formats four authors using
\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{}{}
that is, three authors in the first line and the fourth, in a separate line. The required formatting can be achieved by changing the above line to
\@titleauthors{i}{ii}{}\@titleauthors{iii}{iv}{}
or to
\@titleauthors{i}{}{ii}\@titleauthors{iii}{}{iv}
for larger separation between the authors (in my code I chose the first possibility, but you can use the one that suits you better).
For ecomony in the code, this can also be done by patching (using etoolbox, for example) the \@maketitle command:
\documentclass[preprint,10pt]{sigplanconf}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@maketitle}{\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{}{}}{\@titleauthors{i}{ii}{}\@titleauthors{iii}{iv}{}}{}{}
\makeatother
\begin{document}
\title{My Title}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\authorinfo{Fname Lname}
{school}
{email@edu}
\maketitle
\begin{abstract}
\end{abstract}
\end{document}
To get bigger separation, one would use
\makeatletter
\patchcmd{\@maketitle}{\@titleauthors{i}{ii}{iii}\@titleauthors{iv}{}{}}{\@titleauthors{i}{}{ii}\@titleauthors{iii}{}{iv}}{}{}
\makeatother
The reason is that this template is a CLASS file *.cls so you need to add it in your *.tex document as:
\documentclass[conference]{sig-alternate}
where sig-alternate is the required *.cls file that needs to be in the working directory of your project. You can create this class file sig-alternate.cls by copying the code from the link: http://conferences.sigcomm.org/sigcomm/2015/doc/sig-alternate-10pt.cls and pasting it in any text editor and saving it as *.cls. I hope you got my point.
Best Answer
The LaTeX class
acmartcompiles the code.