\documentclass[a4paper]{article}
\usepackage[english]{babel}
\usepackage{blindtext}

\makeatletter
\renewcommand{\paragraph}{\@startsection{paragraph}{4}{0ex}%
   {-3.25ex plus -1ex minus -0.2ex}%
   {1.5ex plus 0.2ex}%
   {\normalfont\normalsize\bfseries}}
\makeatother

\stepcounter{secnumdepth}
\stepcounter{tocdepth}

\begin{document}
\tableofcontents
\section{Section title}
\subsection{Subsection title}
\subsubsection{Subsubsection title}
\paragraph{Paragraph title}
\blindtext

\end{document}