4e requires the same variety of characters. Fighter/ Wizard/ Cleric/ Thief has transformed into Defender/ Controller/ Leader/ Striker, but the concept is the same--a party can best handle a wide variety of challenges if there is a wide variety of skills represented.
If you honestly believe that an assumption of four specific classes and an assumption of four categories of classes are "the same variety", then I really don't think I can say anything to persuade you.
Still I may as well make a simply mathematical comparison. Assuming a four person party that adheres strictly to the "requirement" and a total of five classes for each 4E role... Under the 3E requirement, every party would have exactly the same class composition. Under the 4E requirement, there would be 625 different valid party combinations. Bending these requirements would give both editions a roughly equal amount of an increase to party variety, so the advantage always goes to 4E. I would never call that an equal amount of variety.
For instance, isn't the fact that monster defenses scale past character attacks at high levels make leaders a necessity? And Strikers are needed if ever the party runs into a Solo, and heaven help a party without a Controller if they're swarmed by 20 minions...the list goes on.
The defenses issue can be answered by purchasing certain feats (that is a different debate, though). Strikers are not needed for a solo, they just make fighting a solo a bit easier. A team that gets swarmed with 20 minions is just fine without a controller, as long as the other classes have a few area of effect powers of their own (and there are a number of good powers available for Defenders and Strikers in this regard). Sure, certain roles would be really useful in these situations, but they are not even close to being necessary.
Compare this to something like trying to get past a Wall of Force in 3E. If you don't have a Wizard or Cleric (or equivalent), and you absolutely need to get past a Wall of Force, then you are totally helpless. There is literally
nothing you can do except give up. A team of non-casters basically have to hope that their DM will take pity on them and not present them with such challenges, even though such a challenge may be trivial if there was a Wizard or Cleric in the party. It is only because 3E reveled in such absolutes of ineffectiveness that it had classes you could call "requirements", and 4E has nothing of the kind.
I never saw a 3e campaign that absolutely couldn't be finished without a Wizard--that would be poor game design, because even if the party had a Wizard, what if he or she was killed or incapacitated? Throw in the towel? Instead, I saw parties that had to think outside the box when the Wizard wasn't present, (or Rogue, or Cleric, or Fighter, or Druid/Spellsword/Thaumaturge, etc) . And I imagine I'll see the exact same situation in 4e.
Sure, you could easily have a 3E campaign that didn't have a wizard. It just depended on a DM deliberately avoiding all the countless traps, monsters, and magic spells that completely destroy any team that couldn't deal with them. This really
is a case of bad game design that is merely being compensated for by decent DMing.
The Wizard in and of itself is not the problem in 3e--it all comes down to player error, player selfishness, and/or poor DMing or campaign design. And 4e, for all its balance, can't possibly protect a party from those things.
Honestly, the Wizard really wasn't quite the problem in of itself in 3E. It just made the problem a
lot worse, and other aspects of game design added on top of that. I guess it could be said that player selfishness is the real problem, but the Wizard class and 3E's overall design made it very easy for a player to mistake selfishness for playing the game as intended.
Also, I do think it is possible for good game design to help things like player error, player selfishness, poor DMing, and bad campaign design. If the rules themselves are more clear, the reasoning behind the rules is more transparent, and it is easier for DMs to make fun and well-balanced adventures, then it would help address many of those issues (since many are caused by confusion regarding bad rules and a high learning curve for the game as much as they are by any other factor). I do believe that 4E has accomplished that as much as you could reasonably ask it to.