This hypothetical Sith "Agent" class would answer directly to Sith Empire superior officers. They would favor lightweight armor and weaponry to achieve higher levels of speed and maneuverability. This class gets in -- potentially using cover -- achieves the mission goal and immediately gets out of dodge. Story missions would be vastly different from any of the other classes, dealing with all the typical spy and assassin stories we're familiar with, only in a Star Wars setting.
Finally, an agent/assassin class offers the opportunity for BioWare to supply frothing HK-47 fans with the ability to play a hunter killer droid. Statement: In the event of said opportunity being squandered, I will hunt down and dispose of the responsible parties' frail, organic forms with excessively lethal force. Addendum: I may also weep.
The last pairing really have been the most difficult to determine. For a long while I considered a pet class, but with the companion system it seems a little redundant. Instead, what finally dawned on me is that the likely solution is yet another blended class with many varying builds. I'll start with the Republic variant: the mechanic.
Decent with a blaster, but far better at field repairs and support, the mechanic focuses on many fields of utilitarian expertise. One such area would be basic astromech or protocol droids, while another could involve heavy crowd control through various pieces of deployable equipment like turrets, or cover for the classes that can use it. As BioWare has hinted at in regards to high-level PvP and mission play, the mechanic would have the option to invest in healing and buffing abilities, as well as some de-buffing. Come to think of it, this is essentially the iconic Chewie class.
While I feel pretty confident in my Republic picks, it seems like the last Empire class will allow BioWare to really get creative. It's doubtful that we'd get a mechanic polar opposite. I've seen slaver mentioned around the forums, and it's a good idea but the crux of it leans too heavily on pets, which everyone in this game will kind of have via companions. Simple droid pets are one thing, but slavers would need a plethora of abilities centered around pets.
Would BioWare go with only one pet class, further risking a skewed faction ratio than already exists with Jedi? I somehow doubt it. Until some more hints are dropped, that last class pairing is really anyone's guess. For instance, it'd be easy enough to look at the Sith Empire allegiance page and notice the "quick notes" which say their population is severely lower than the Republic's. Could this mean the fourth class is some kind of droid-based concept? Maybe, or maybe not.
Then again, guessing is part of the fun. Something tells me that even after we know all the classes, even more questions will arise. Racial options haven't even been officially revealed yet! Ah, BioWare, how you love to tease.