If roles were separated from classes, how many are there?
		
		
	 
Traditionally, 4:  meat shield, heal-bot, skill monkey, and deus ex magica.
	
	
		
		
			But did Wizards of the Coast identify the correct roles in 4th edition?
		
		
	 
The identified a workable set of 4 roles.  I'm certain there could be others.  In particular, the Controller is a shakily-defined role.
	
	
		
		
			Did they shoehorn roles into a set of four (4) because there are four traditional D&D classes?
		
		
	 
Certainly.  
	
	
		
		
			Is dealing "damage to multiple foes at once" inherently in the same role as weakening (handing out debuffs), confusing, or delaying your foe? Or is this just a mish-mash of "things wizards do"?
		
		
	 
The latter, it seems kinda like a grandfather clause to let wizards keep some of their excessive versatility and power.
Roles work pretty well.  The controller role could certainly be re-examined, perhaps cut into more workable chunks.  For instance, you could break a 'blaster' role specializing in multi-target damage (like a 4e sourcerer or evoker) out of controller, leaving a role specialized in condition-infliction and action denial.
Another option is to relate roles to the 'three pillars.'  There could be a variety of systematic aproaches.
You could take the existing 4 roles and give them corresponding functions in each of the pillars.  
Combat:
Defender:  Marks/blocks/soaks damage
Leader:  Buffs/heals/preserves actions
Striker:  high-damage,skirmishing
Controller:  blasting/area-denial
Exploration:
Defender:  Directly removes obstacles, takes risks
Leader:  Provides bonuses/removes penalties
Striker:  Spots trouble, directly removes enemies/traps
Controller:  Provides knowledge, changes circumstances
Interaction:
Defender:  Counters threats of violence, presents a position of strenght/status, provides access.
Leader:   Clarifies positions, offers compromises, smooths over objections 
Striker:  Engages in high-rist/reward strategies (deception, blackmail, etc)
Controller:  Gains information, alters dynamics
Or,  you could have different roles in each pillar:
Combat:  Defender, Striker, Leader, Controller
Exploration:  Scout, Navigator, Expert, Trail-breaker
Interaction:  Investigator, Researcher, Negotiator, Authority
Different classes could have different combinations of roles.  Or some roles could be determined by class and others by theme.