I am using \externalfigure to show a picture at the desired scale. Instead
of providing a number for the width parameter I have made a definition with
\def. When I run in mkiv this I get the text "4t" alongside the picture.

\def\DesignSize#1{3.5cm/4*#1}%
\starttext
\externalfigure[hacker.jpg][width=\DesignSize{4}]
\stoptext

In mkii is perfect.

I've also tried

\define[1]\DesignSize{3.5cm/4*#1}

with the same result.