How to change spacing in the ACM conference template?

Just set to 2 authors per row:

settopmatter{authorsperrow=2}

Now it fits.

Edit: Second Option

An option is to put the two guys from the same institution in one cell, and perhaps abbreviate the huge institution name

Up to you.

documentclass[sigconf]{acmart}
usepackage[portuguese]{babel}

settopmatter{authorsperrow=2}

AtBeginDocument{%
  providecommandBibTeX{{%
    normalfont Bkern-0.5em{scshape ikern-0.25em b}kern-0.8emTeX}}}

setcopyright{acmcopyright}
copyrightyear{2018}
acmYear{2018}
acmDOI{10.1145/1122445.1122456}

acmConference[Woodstock '18]{Woodstock '18: ACM Symposium on Neural
  Gaze Detection}{June 03--05, 2018}{Woodstock, NY}
acmBooktitle{Woodstock '18: ACM Symposium on Neural Gaze Detection,
  June 03--05, 2018, Woodstock, NY}
acmPrice{15.00}
acmISBN{978-1-4503-XXXX-X/18/06}

begin{document}

title{Paper ACM Template}

author{Denis B. Oliveira}
email{[email protected]}
authornotemark[1]


author{Ricardo R. de Oliveira}
email{[email protected]}
authornotemark[1]
affiliation{
  institution{IFSULDEMINAS}
  city{Poços de Caldas}
  state{MG}
  country{Brasil}
}

author{Ricardo F. Vilela}
email{[email protected]}
authornotemark[2]
affiliation{
  institution{Universidade de São Paulo}
  city{São Carlos}
  state{SP}
  country{Brasil}
}

author{Victor H. S. C. Pinto}
email{[email protected]}
authornotemark[3]
affiliation{
  institution{Zup Innovation}
  city{São Paulo}
  state{SP}
  country{Brasil}
}

author{Roberto N. Ungarelli}
email{[email protected]}
authornotemark[4]
affiliation{
  institution{Oceano Azul TI}
  city{Brasília}
  state{DF}
  country{Brasil}
}

renewcommand{shortauthors}{Denis B. Oliveira, et al.}

begin{abstract}
textbf{Background}: ...
end{abstract}

begin{CCSXML}
<ccs2012>
   <concept>
       <concept_id>10002944.10011123</concept_id>
       <concept_desc>General and reference~Cross-computing tools and techniques</concept_desc>
       <concept_significance>500</concept_significance>
       </concept>
   <concept>
       <concept_id>10002944.10011123.10011674</concept_id>
       <concept_desc>General and reference~Performance</concept_desc>
       <concept_significance>500</concept_significance>
       </concept>
 </ccs2012>
end{CCSXML}

ccsdesc[500]{General and reference~Cross-computing tools and techniques}
ccsdesc[500]{General and reference~Performance}

keywords{Teste de desempenho, Plataforma como serviço, Sistema como serviço, Avaliação Experimental, Computação em nuvem}

maketitle

section{Introdução}
...

%bibliographystyle{ACM-Reference-Format}
%bibliography{references.bib}

end{document}

Edit: Third Option

If you really want to do this, you can try setting

makeatletter
%def@authorfont{normalfontsmall} %change font. don't do this.
%def@affiliationfont{normalfontsmall} %change font. don't do this.
defauthor@bx@sep{0pc}
makeatother

before begin{document}.

Then it fits. But be aware that all changes you make are liable to come to nought: if the editor wants a 2x2 setup with one author in the last line or a 2x2 option with authors of the same institution in the same cell, he will.