Is the Canon EF 400mm f/4 DO IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 400mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Canon EF 400mm f/4 DO IS USM Lens set to 400mm f/4.5 sharper than the Canon EF 400mm f/4 DO IS II USM Lens set to 800mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.