stopping down a lens beyon a stop or two (in a futile attempt to gain some DoF) will seriously degrade image quality (no matter whether 4x5 or 35 mm format).
In reality, I suppose that it's not the format which makes the difference but it's the overall magnification from subject to final print size, the film format just being an intermediate step in the process. The greater the magnification, the more obvious the signs of diffraction.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.