It is massive.

The difference in the AF system outweighs any perceived benefit that you may think the 7D has.
The quality of the pictures directly out of the 1D IV are superior to the 7D’s directly out of the camera. The 1D IV has much better contrast than the 7D. You can get close to the same image quality with the 7D if you are willing to work at it in post.
I thought it was a worthy upgrade when the 7D was $1,600 and the 1D IV was $4,500.