Well if the 4.5 is half the size of yours that would make it a 9 meter! But with a 411cm mast that's pretty doubtful
I'm guessing that the shape of your sail and the 4.5 are quite different, so it's difficult to accurately compare their sizes.
This is probably a pretty old sail? Does it have any full battens; ones that go all the way from the leech (trailing edge) of the sail all the way to the mast?
One minor adjustment would probably be in the mast sleeve - it's probably curved so it's not quite a triangle.
If it's old enough, it might be marked with the area in square feet. Then you can convert using: sq ft x 1.1 = sq meters
You can always just sell it with a picture and the mast & boom measurements.
Drew