Step 2. I make the cover in its own document. (I confess I don't remember why I chose to do this)
Step 3. And then in a different document, I perform imposition to map the pages to the folds.
\definepapersize[sheet][width=99mm,height=210mm]
\setuppapersize[sheet][A4, landscape]
\setuppaper[nx=3, ny=1, dx=0mm, dy=0mm] % must come before \setuparranging!
\setuplayout[page][location=middle, marking=on]
\setuparranging [XY]
\setuplayout
[backspace=0pt,
topspace=0pt,
width=middle,
height=middle,
location=middle,
header=0pt,
footer=0pt]
\starttext
\filterpages[../pdf/mobilePamphletInterior.pdf][4][width=0pt]
\filterpages[../pdf/mobilePamphletCover.pdf][2][width=0pt]
\filterpages[../pdf/mobilePamphletCover.pdf][1][width=0pt]
\filterpages[../pdf/mobilePamphletInterior.pdf][1,2,3][width=0pt]
\stoptext