Using closeup filters on LF lenses

I've been trying to compensate for my short bellows (don't laugh) I screwed on a 4x closeup filter to my lens and experienced some vignetting. Does the use of closeup lenses reduce the coverage of the lens? I was not using any camera movements in the shot and have not experienced the same with the 2x closeup filter I originally tried.

*snicker, snicker*

Hummmph - I wonder if that might be a benefit - like the look of a plastic lens .... Of course there is always the pin hole.