I am not an expert, but have owned both.
As best as I can tell, the 4000 has a much better screen layout and it more intuitive with the buttons.
As for what I think - but am not sure on. The 2000 can do most things up to 1995 with a pathfinder cartridge. But, it does not do OBD2. The 4000 can do OBD2.
I have a 4000E I picked up - works nice.