Is the Canon EF 600mm f/4L IS II USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 840mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Canon EF 600mm f/4L IS II USM Lens set to 840mm f/5.6 sharper than the Canon EF 400mm f/2.8L IS II USM Lens set to 560mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.