documentclass{article}
usepackage[DIV9]{typearea}
usepackage{lipsum}

usepackage{letltxmacro}
makeatletter
LetLtxMacro{ltx@subsection}{subsection}
renewcommand{subsection}[2][]{ltx@subsection[#1]{protectparbox{4in}{#2}}}
makeatother

begin{document}
section{Heading}
subsection{Very very very very very very long subheading with no single line wider than 4 inches}
lipsum
end{document}