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.