With tocloft (not recommended with a KOMA-Script class) the number width for the entries in TOC is set by the third argument of \cftindents. Assumnig the number width should be 50pt
\cftindents{section}{0pt}{50pt}
But you will get the same result using
\setlength\cftsecindent{0pt}
\setlength\cftsecnumwidth{50pt}

Code:
\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage{tocloft}
\renewcommand\thesection{\arabic{section}}
\cftsetindents{section}{0pt}{50pt}% <- changed
\cftsetrmarg{2cm}
\renewcommand{\cftdot}{}
\begin{document}
\tableofcontents
\addchap*{Chapter 1}
\section{Section 1}
\section{Section 2}
\section{Section 3}
\section{Section 4}
\end{document}
Suggestion without tocloft:
\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\renewcommand\thesection{\arabic{section}}
\RedeclareSectionCommand[
tocindent=0pt,
tocnumwidth=50pt,
]{section}
\RedeclareSectionCommands[
toclinefill=\hfill
]{section,subsection,subsubsection,paragraph,subparagraph}
\makeatletter
\renewcommand{\@tocrmarg}{2cm}
\makeatother
\begin{document}
\tableofcontents
\addchap*{Chapter 1}
\section{Section 1}
\section{Section 2}
\section{Section 3}
\section{Section 4}
\end{document}
Because of a comment below: The headings of TOC, LOF and LOT are chapters by default (with a book class). So if you want to modify the font of the TOC title and the skip below the TOC title, you can use eg.
\BeforeTOCHead[toc]{%
\addtokomafont{chapter}{\normalsize}
\RedeclareSectionCommand[afterskip=1sp]{chapter}%
}
If you remove the optional argument, then the the titles of the lists (LOF and LOT) will change in the same way.