The AF locking and tracking ability is on par maybe a bit better than 1D4 that I am currently own. Yes, 5dIII loses the reach but reach is not everything either. 5D III will allow you to shoot in poor lighting conditions(cloudy, early morning, dawn etc) since the file quality is real nice at higher ISO settings.
You can buy a used 1D4 for about the price of 5DIII, so it is your call. For my type of shooting, I prefer walking with my gear ready for any sudden action, so I prefer lighter body.
All with 5D III + ef400/5.6L