Say you want to play a Swashbuckler. Maybe not a very good example, because until Complete Warrior, such a build was hard to pull off in 3E. But in 4E, it can't really be done at all at the moment. Ranger needs two weapons and is Str-based. Rogue needs to sneak or outflank which is certainly not swashbuckler-ish. Fighter is Str-based. The avenger might be close, but its a religious nut and works best with a maul.
Oh, for crying out loud. I hate it when people don't really try.
Look... What's a swashbuckler? In the context of D&D, you need A) little or no armor, B) no shield, C) wields light blades, D) may or may wield two weapons (rapier and dagger is not uncommon), E) highly mobile, F) relies on a high Dexterity and a high Charisma.
In D&D 4E, that's a rogue, pure and simple. Choose Artful Dodger. Pick powers that keep him moving (like Deft Strike, King's Castle and Tumble), pick powers that have extra bonuses based on Charisma modifiers (like Sly Flourish, Posistioning Strike and Bait and Switch), and pick powers that inflict conditions that grant combat advantage (like Dazing Strike, Blinding Barrage or Easy Target) so that you have to rely on flanking less.
Roleplay him as a cocksure, sarcastic, loudmouthed swordsman who thinks he's the best in the world, and voila, you've got a swashbuckler.
Assassin is even easier... Anyone who kills for money, and is sneaky is an assassin. Period.
Be any class you like (but especially strikers). Take stealth as a skill. Choose skills that deal as much damage to a single target as you possibly can. Done. The Rogue class even has a Paragon Path called "Shadow Assassin".