It's four new souls, one for each calling. For those unfamiliar with Rift a "calling" is your base class - warrior, mage, cleric or rogue - and each calling has a selection of "souls" you can choose between.
As an Ascended (player character) you have the unique ability to choose three souls to use at a time, each soul having its own talent tree and associated skills. So a Rogue could have Bard/Assassin/Nightblade or Marksman/Ranger/Bard or any of a bunch of other combinations.
So adding four souls actually adds a lot more than it might appear at first glance, since you'll be able to combine a new soul with any two of the original ones for a brand new combination.
The expansion looks to be adding a bunch of what players have been hoping for, including the long requested capes. I think I'll have to resubscribe some time soon.