documentclass{article}
usepackage{titlesec}

setcounter{secnumdepth}{4}

titleformat{paragraph}
{normalfontnormalsizebfseries}{theparagraph}{1em}{}
titlespacing*{paragraph}
{0pt}{3.25ex plus 1ex minus .2ex}{1.5ex plus .2ex}

begin{document}

section{Test Section}
test
subsection{Test Subsection}
test
subsubsection{Test Subsubsection}
test
paragraph{Test Modified Paragraph}
test

end{document}

documentclass{article}
usepackage{titlesec}
usepackage{hyperref}

titleclass{subsubsubsection}{straight}[subsection]

newcounter{subsubsubsection}[subsubsection]
renewcommandthesubsubsubsection{thesubsubsection.arabic{subsubsubsection}}
renewcommandtheparagraph{thesubsubsubsection.arabic{paragraph}} % optional; useful if paragraphs are to be numbered

titleformat{subsubsubsection}
  {normalfontnormalsizebfseries}{thesubsubsubsection}{1em}{}
titlespacing*{subsubsubsection}
{0pt}{3.25ex plus 1ex minus .2ex}{1.5ex plus .2ex}

makeatletter
renewcommandparagraph{@startsection{paragraph}{5}{z@}%
  {3.25ex @plus1ex @minus.2ex}%
  {-1em}%
  {normalfontnormalsizebfseries}}
renewcommandsubparagraph{@startsection{subparagraph}{6}{parindent}%
  {3.25ex @plus1ex @minus .2ex}%
  {-1em}%
  {normalfontnormalsizebfseries}}
deftoclevel@subsubsubsection{4}
deftoclevel@paragraph{5}
deftoclevel@paragraph{6}
defl@subsubsubsection{@dottedtocline{4}{7em}{4em}}
defl@paragraph{@dottedtocline{5}{10em}{5em}}
defl@subparagraph{@dottedtocline{6}{14em}{6em}}
makeatother

setcounter{secnumdepth}{4}
setcounter{tocdepth}{4}

begin{document}

tableofcontents
section{Test Section}
test
subsection{Test Subsection}
test
subsubsection{Test Subsubsection}
test
subsubsubsection{Test Subsubsubsection}
test
paragraph{Test Paragraph}
test
subparagraph{Test Subparagraph}
test

end{document}

newcommandsubsubsection{@startsection{subsubsection}{3}{z@}%
                {-3.25ex@plus -1ex @minus -.2ex}%
                {1.5ex @plus .2ex}%
                {normalfontnormalsizebfseries}}
newcommandparagraph{@startsection{paragraph}{4}{z@}%
                {3.25ex @plus1ex @minus.2ex}%
                {-1em}%
                {normalfontnormalsizebfseries}}

documentclass{article}
makeatletter
renewcommandparagraph{@startsection{paragraph}{4}{z@}%
            {-2.5ex@plus -1ex @minus -.25ex}%
            {1.25ex @plus .25ex}%
            {normalfontnormalsizebfseries}}
makeatother
setcounter{secnumdepth}{4} % how many sectioning levels to assign numbers to
setcounter{tocdepth}{4}    % how many sectioning levels to show in ToC

begin{document}
tableofcontents
section{A}
subsection{B}
subsubsection{C1}
paragraph{D1}
paragraph{D2}
subsubsection{C2}
end{document}

documentclass[article,oneside]{memoir}
setcounter{secnumdepth}{5} % Note that part is -1 level !
setcounter{tocdepth}{5}
begin{document}
begingroup
letclearpagerelax
letnewpagerelax
tableofcontents*
part{Part}
endgroup
chapter{Chapter} Text.
section{Section} Text.
subsection{Subsection} Text.
subsubsection{Subsubsection} Text.
paragraph{Paragraph} Text.
subparagraph{Subparagraph} Text.
end{document}

newcommand{subsubsubsection}[1]{paragraph{#1}mbox{}\}
setcounter{secnumdepth}{4}
setcounter{tocdepth}{4}

subsubsubsection{Navigator}

RedeclareSectionCommand[runin=false,afterskip=1ex,afterindent=false]{paragraph}

setcounter{secnumdepth}{paragraphnumdepth}

RedeclareSectionCommand[indent=0pt,runin=false,afterskip=1ex,afterindent=false]{subparagraph}

DeclareNewSectionCommand[style=section,level=numexpr subparagraphnumdepth+1relax,runin=false,beforeskip=1ex,afterskip=0pt,afterindent=false]{subsubparagraph}