Writing Roman numbers in Equation

If you don't want to do the conversion,

\documentclass{article}
\usepackage{amsmath}

\newcommand{\RN}[1]{%
  \textup{\uppercase\expandafter{\romannumeral#1}}%
}

\begin{document}
\[
\RN{4}+\RN{12}=\RN{16}
\]
\end{document}

If you also don't want to do the arithmetic

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\RN}{m}
 {
  \textup{ \int_to_Roman:n { #1 } }
 }
\ExplSyntaxOff

\begin{document}
\[
\RN{4}+\RN{12}=\RN{4+12}
\]
\end{document}

I understand, that you are expecting a plain TeX solution, as it is suggested by tex-core tag, hence

$$
1+1=2
\eqno{\rm IV}
$$

\bye

For the sake of completeness. The conversion to roman numeral will be done automatically.

\documentclass[varwidth,math]{standalone}% change it back to your own document class

\newcounter{x}
\def\ToRomanEmpire#1{\setcounter{x}{#1}\Roman{x}}


\begin{document}
\[
    \ToRomanEmpire{1999} + \ToRomanEmpire{15} = \ToRomanEmpire{2014}
\]
\end{document}