Is the Canon EF 24-105mm f/4L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 105mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 24-105mm f/4L IS USM Lens set to 105mm f/4.0 sharper than the Nikon 18-105mm f/3.5-5.6G AF-S VR DX Nikkor Lens set to 50mm f/5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.