Game shoots up in difficulty pretty good as you go further, both for dungeons and elite quests, it was a big leap after the stuff on Kass and the first flash point (which I don't believe even suggests a full group), though.
I havent done anything with 4 players though, I'd rather just have us use the companions for slots 3 and 4 and keep on keepin' on as co-op until it becomes impossible.
Think these are all supposed to have heroic modes, but that's at cap of course.
Which class has the most interesting storyline?
Everyone is going to tell you Agent, and they will all be correct. Has a bit if a slow start but after it ramps up it's probably the most interesting fare in a Bioware game since the old days, and I mean Infinity Engine days, KOTOR's Jedi stuff didnt do much for me as I'm not into Star Wars mythos normally. Agent, however is completely rad and should be revisited as a full darn game in the future.
It's just sleek and cool with better characters and twists and the spy setting makes better use of the light/dark dialogue than the rest, IMO.
Bounty Hunter is also very nice, and I appreciate how it literally plays to it's namesake. It makes for fun little episodic bits.