\documentclass[a4paper,12pt,openany,oneside]{article}
\usepackage{amsmath,amssymb,amsthm,tabularx}
\usepackage[left=2.5cm,right=2.5cm,top=.5cm,bottom=1cm,nohead]{geometry}
\usepackage{roundbox,graphicx,framed}
\usepackage{makebox}
\usepackage{tcolorbox}
\usepackage{wasysym}
\usepackage{enumerate}
\usepackage{float}
\usepackage{xepersian}
\usepackage{dashrule}
\usepackage{ptext}
\settextfont[Scale=1.1]{XB Niloofar}
\setdigitfont[Scale=1.1]{PGaramond}

%\DeclareMathSizes{12}{15}{9}{8}

\PersianMathsDigits
\defpersianfont\nas[Scale=2.5]{IranNastaliq} 
\newcommand*{\Scale}[2][4]{\scalebox{#1}{$#2$}}%
\newcommand*{\Resize}[2]{\resizebox{#1}{!}{$#2$}}%
%\usepackage[Kashida]{xepersian}
\begin{document}
\thispagestyle{empty}


\begin{align*}
{\normalsize f(x)=
 \begin{cases}
a[x+2] & \  x > 4 \\ 
[\frac{x}{3}]+b  & \ x=4 \\ 
\frac{x^2 -16}{x-4} & \  x<4 
\end{cases}
}
\end{align*}
\end{document}