Have owned both the 1D3 and 1D4 I can honestly say that for most uses the 1D4 has very little over the 1D3. If you're a sports shooter making your living shooting professional sports events, then the 1D4 might make sense. But in terms of image quality, build quality, general AF performance, etc. the 1D3 is more than enough camera for most shooters, myself included.
The 1D3 compared to the 50D, there's no comparison. I owned a 50D and 1D3 at the same time, they're worlds apart... but that doesn't mean the 50D isn't a great body because it is. But if you're into shooting action the 1D3's AF system is light years ahead of the 50D's AF system.