The Assassin will need to be behind for most special attacks as well as stealth.
The Ranger will need to be behind and a little further out so that they can do both melee & ranged. Getting the range right consistantly will be frustrating.
Both the Swashbuckler and Brigand will require rear, front or flank posistions depending on which CA is being used. Less depenedence on stealth though.

From a boxing perspective though, none of these are insurmountable. If you have a tank that can spin the mobs (can only think of Monks here via AA, but I'm sure there are more) then the 2 rogues are fairly easy. If you are creative with your macros, the assassin can fire off stealth attacks fairly easily without being micromanaged. IMO, I'd think the Ranger will be hardest depending how small or large your group is and how well your independant movement keys are setup.