With modern lenses, the image circle stays the same usually but falloff of illumination increases as you open up, so on 4x5" you might find that light still gets out to the corners with the lens wide open, but the corners are darker relative to the center.
If you have a camera with a groundglass that has cut corners, you can see what's going on with this. When the lens is wide open, you might be getting some mechanical vignetting of the aperture from the lens barrel (and shade, if you're using a shade) at the edge of the image circle. Even if there is no mechanical vignetting, the aperture may look like a football (an American football, that is) rather than a circle, and that's the main thing that's causing the corners to be darker. As you stop down, there will be less vignetting of the aperture at the edge of the image circle. When you can see the whole aperture and it looks like a circle, you should have pretty even illumination.
The best solution for that is to use a center filter usually, but sometimes that is going to mean you don't have enough light to make the photograph you want (the CF will cost you about 1-1/2 stops usually, and the lens usually should be around f:16-22 with the CF), so you might shoot without the CF and correct it by burning in the center of the image in the darkroom.