1) The return of the old panic attack
2) No random crits and a lower clip size (3-4 shells), but it has a faster reload and firing speed and does mini-crits on invisible spies.
3) Has a more flank oriented style, so a smaller clip but it has a fixed shot pattern and you move slightly faster.
4) The most offensive powerhouse you can think of, Bigger clip, Faster firing speed, more damage, but a very slow reload.
These aren't objective, just my opinions on a weapon Valve probably wont even add.