Actually, it's more complicated, which is why I asked the question.
Everyone who answered assumed a flat bottom, which was not the case.
"Scope" gives the angle between the rode and the bottom, but if the bottom
is sloped, it's not the same as if it's flat. In this case, the actual
scope would be something less than 4:1 since the bottom was sloping out.
(draw a diagram to see what I mean...)

Ahh, I didn't know that. I thought the depth under the boat would be
irrelevant and that you threw that in there to add some confusion. The
things I don't know ...

Problem is, all Canadian charts now show depths in meters. Yuk!

Wonder if electronic charts will allow you to switch between units?