Is the Canon EF 300mm f/4L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 420mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Canon EF 300mm f/4L IS USM Lens set to 420mm f/6.3 sharper than the Canon EF 400mm f/5.6L USM Lens set to 400mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.