Exam package: How can I stick part and solutionbox together?

I’m using the exam class and have the problem with the parts enviroment in combination with the solutionorbox enviroment. The part and the corresponding solutionorbox should be on the same page. Here is an example that demonstrate the problem:

documentclass{exam}
usepackage{blindtext}
begin{document}
begin{questions}
titledquestion{Farben}
begin{parts}
part[4] Question 1 blindtext
begin{solutionorbox}[16cm]
    blindtext
end{solutionorbox}
part[4] Question 2 blindtext
begin{solutionorbox}[10cm]
    blindtext
end{solutionorbox}
end{parts}
end{questions}
end{document}

I have tried to modify the part command by using etoolbox package but it does not work with:

preto{part}{parnobreakvfilpenalty 9999vfilnegvtopbgroup}

This is the part definition in the exam.cls:

defpart{%
  @bonusfalse
  process@part
}%

How can I add a newpage if needed that the question and the space for a solution is on the same page without adding the newpage?

documentclass{exam}
usepackage{blindtext}
usepackage{etoolbox}
begin{document}
begin{questions}
titledquestion{Farben}
begin{parts}
preto{part}{parnobreakvfilpenalty 9999vfilnegvtopbgroup}
part[4] Question 1 blindtext
begin{solutionorbox}[16cm]
    blindtext
end{solutionorbox}
egroup
part[4] Question 2 blindtext
begin{solutionorbox}[10cm]
    blindtext
end{solutionorbox}
egroup
end{parts}
end{questions}
end{document}