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/6.3 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/6.3 sharper than the Canon RF 600mm F4 L IS USM Lens set to 600mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.