My thinking is simply this:

If the negative is in a carrier, and the light source is adjusted so that the entire negative is evenly illuminated, and the lens has adequate coverage for that size negative, then moving the easel will not result in vignetting, rather the straight edge of the negative carrier will be "shown" on the paper, including the corners of the negative, since they are all within the cone of view for the lens. Only if the lens coverage or the circle of illumination are inadequate will the corners be vignetted. The OP is using a 4x5 enlarger, so illumination coverage is not his problem. Therefore, if he has vignetting, his enlarging lens is not seeing the corners of his negative as far as I can tell.

I can certainly move my easel off to one side and still print the corners of my negatives, even if they end up in the middle of the paper! For that matter, one should be able to use a smaller amount of enlargement and print all four edges of the negative carrier on the printing paper without vignetting at the corners. Sometimes I intentionally do this so I can trim the print exactly to the outside of the image area easily. I'm enlarging 4x5 with 150mm lenses and never have clipped corners.