What does The Horned Rat expansion add to the game? How is play different to the original?
The rats are interesting:
They are actually very well balanced to my opinion. While giving more locations for Korne to attack, the rat king can easily kill korne's units if he pleases.
They don't really ruin a region considerably faster, but seem to be balanced in their ability to score points.
That said I'd play with the default 4 powers if I only had 4 people.
Additionally, the expac adds more difficult old world cards, as well as alt decks for the normal powers.
My players haven't liked the alt-decks, except the alt-korne one (you're supposed to use all of the alt decks or nothing). We haven't tried the more difficult old world cards (which are supposed to make mutual loss much more likely).