\subsubsubsection for scrbook

Update (needs KOMA-Script version 3.26b)

The original answer below is still correct. But there are the new command \RedeclareSectionCommands, the new options runin and afterindent. And there are some options predefined for new section levels using style=section.

Suggestion 1: Declaring new section level subsubsubsection as a displayed title

\documentclass{scrbook}

\DeclareNewSectionCommand[
  style=section,
  counterwithin=subsubsection,
  afterskip=1.5ex plus .2ex,
  beforeskip=3.25ex plus 1ex minus .2ex,
  afterindent=false,
  level=\paragraphnumdepth,
  tocindent=10em,
  tocnumwidth=5em
]{subsubsubsection}
\setcounter{secnumdepth}{\subsubsubsectionnumdepth}
\setcounter{tocdepth}{\subparagraphtocdepth}

\RedeclareSectionCommands[
  level=\numexpr\subsubsubsectionnumdepth+1\relax,
  toclevel=\numexpr\subsubsubsectiontocdepth+1\relax,
  increaselevel
]{paragraph,subparagraph}
\RedeclareSectionCommand[
  counterwithin=subsubsubsection,
  tocindent=12em,
  tocnumwidth=6em
]{paragraph}
\RedeclareSectionCommand[
  tocindent=14em,
  tocnumwidth=7em
]{subparagraph}

\usepackage{lipsum}% only for dummy text
\begin{document}
\tableofcontents
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\subsubsubsection{Subsubsubsection}
\lipsum[1-2]
\end{document}

Suggestion 2: Redefining paragraph as a displayed title:

\documentclass{scrbook}

\RedeclareSectionCommand[
  afterskip=1.5ex plus .2ex,
  afterindent=false
]{paragraph}
\setcounter{secnumdepth}{\paragraphnumdepth}
\setcounter{tocdepth}{\paragraphtocdepth}

\usepackage{lipsum}% dummy text
\begin{document}
\tableofcontents
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\paragraph{Paragraph}
\lipsum[1-2]
\end{document}

---

Original answer

You can declare a new section level subsubsubsection. But then you have to redeclare paragraph and subparagraph to adjust the its levels etc.:

\documentclass{scrbook}

\DeclareNewSectionCommand[
  style=section,
  counterwithin=subsubsection,
  font=\normalsize,
  afterskip=1.5ex plus .2ex,
  beforeskip=-3.25ex plus -1ex minus -.2ex,
  indent=0pt,
  level=4,
  tocstyle=section,
  toclevel=4,
  tocindent=10em,
  tocnumwidth=5em
]{subsubsubsection}
\setcounter{secnumdepth}{\subsubsubsectionnumdepth}
\setcounter{tocdepth}{\subsubsubsectiontocdepth}

\RedeclareSectionCommand[
  level=5,
  counterwithin=subsubsubsection,
  toclevel=5,
  tocindent=12em,
  tocnumwidth=6em
]{paragraph}
\RedeclareSectionCommand[
  level=6,
  toclevel=6,
  tocindent=14em,
  tocnumwidth=7em
]{subparagraph}

\usepackage{lipsum}% only for dummy text
\begin{document}
\tableofcontents
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\subsubsubsection{Subsubsubsection}
\lipsum[1-2]
\end{document}

---

If do not use \paragraph as a runin title, you can redeclare section level paragraph using a positive value for afterskip:

\documentclass{scrbook}

\RedeclareSectionCommand[
  afterskip=1.5ex plus .2ex,
  beforeskip=-3.25ex plus -1ex minus -.2ex
]{paragraph}
\setcounter{secnumdepth}{\paragraphnumdepth}
\setcounter{tocdepth}{\paragraphtocdepth}

\usepackage{lipsum}% dummy text
\begin{document}
\tableofcontents
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\paragraph{Paragraph}
\lipsum[1-2]
\end{document}

The easiest way is to use \paragraph and to redefine its formatting:

\documentclass{scrbook}
\setcounter{secnumdepth}{5}

\RedeclareSectionCommand[
    counterwithin=subsubsection,
    toclinefill=\hfill,
    tocindent=3.5ex,
    tocnumwidth=1ex,
    tocdynnumwidth=true, 
    tocraggedentrytext=true,
    tocentryformat=\fontintoc\itshape,
    tocpagenumberformat=\fontintoc,
    beforeskip=10pt,%
    afterskip=1pt,
    font=\normalfont\itshape
    ]{paragraph}



    \begin{document}

\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\paragraph{Subsubsubsection}
\end{document}

If you want to leave \paragraph and \subparagraph as is, then you need only new level numbers for it and can then define \subsubsubsection:

\documentclass{scrbook}
\RedeclareSectionCommand[level=6,toclevel=6]{subparagraph}
\RedeclareSectionCommand[level=5,toclevel=5]{paragraph}    
\DeclareSectionCommand[
  style=section,
  counterwithin=subsubsection,
  font=\normalsize,
  afterskip=1ex plus .2ex,
  beforeskip=-3.25ex plus -1ex,
  indent=0pt,
  level=4,
  tocstyle=section,
  toclevel=4,
  tocindent=10em,
  tocnumwidth=5em
]{subsubsubsection}

\setcounter{secnumdepth}{\subsubsubsectionnumdepth}
\setcounter{tocdepth}{\subsubsubsectiontocdepth}

\usepackage{lipsum}% only for dummy text
\begin{document}
    \tableofcontents
    \chapter{Chapter}
    \section{Section}
    \subsection{Subsection}
    \subsubsection{Subsubsection}
    \subsubsubsection{Subsubsubsection}
    \lipsum[1-2]
\paragraph{foo} \lipsum[1]
\subparagraph{bar} \lipsum[1]
\end{document}