Exactly!.... but the rigby is too long so it could be cut shorter and then the bullet diameter is only .007" off
I've never tried that combination but it might well work given the small neck diameter difference... at only .004-.007". The actual neck diameter difference will also depend somewhat on the brass thickness. I have modified FCDs for differences in length, but with the same neck diameters. But yes, it's possible, I think. I've reduced the length of the collet on a couple FCDs and it's fairly easy to do. Same theory but in "reverse" of adding spacer (washer) if FCD collet is too short... which also works.
The best way to shorten the collet is to pop the collet out of the die body with a brass rod. Then chuck the collet (finger end) in a lathe or even a large drill press will do. Minimize the runout... it doesn't have to be perfect but the closer to true the better of course. The collet is fairly hard so a diamond file might work but a small hand held grinder or drum sander is much easier. Just touch the base end (the end that contacts the shell holder) of the collet while the collet is turning and "sneak" up on best length by trial and error. Since the Rigby is only about .006-.007" longer,
it won't take much!! Just a touch at a time!! When checking during the process, realize the Rigby neck diameter is very slightly smaller by between .004-007" so the actual compression of the collet on the slightly larger 404 neck may not work or feel exactly like on the 416, but it may work well enough to provide a quality crimp if used with care on how much compression to use. Also, it's a good idea to watch the fingers compress and adjust die accordingly. If it doesn't work, the worst case would be a short Rigby FCD where you might have to add a thin spacer (washer) on top of the shell holder for using on the Rigby. The obvious question then becomes, is the 416 Rigby FCD available?
Pic of FCD with the collet removed from the die body.