You kind of do you kind of don't.
4e is kind of a mix of classless and class based systems.
I think 4E is decidedly a class based system. It basically has two "tiers" of classes:
- Role. This describes what you do on the battlefield.
- Class. This describes the specifics of what you do on the battlefield, but also the stuff outside of combat. What skills do you possess? There is a difference between having Athletics, Acrobatics and Thievery and having Arcana, History and Religion as class skills, even without ever looking at what role or powers you have.
The Power Source is also very important, since it creates a theme for your powers that is consistent with the source. Arcane spells look different from divine prayers or martial exploits.
The Class gives you features and powers, both representing how you do stuff.
Some of the differences in classes become notable when you look at the class features alone. Compare a Shielding Swordmage to a Fighter. The shielding swordmage reduces damage inflicted if an enemy doesn't attack him, the Fighter deals extra damage if an enemy doesn't attack him.
Other things are more reliant on powers. A Wizard has a lot of ranged powers, similar to a Warlock. But the Warlock attacks mostly single enemies, often with some extra effect that lets him move, escape his enemy or avoid his attacks, while the Wizard deals damage to areas.
Classes in some games (including D&D 3E) are often based on using very different magical subsystems. Well, it wasn't that many in 3E.
We basically had weapon users and spell users, with the spell users further differentiated as "spontaneous" or "prepared". There were also some subtle and not-so-subtle differences. But looking merely at spell slots, Cleric and Wizard weren't all that different. But their
power spell lists were very different in certain matters.
Basically, one of the biggest differences in classes was that they used variant "power" systems or resource management models.
The next difference where class specific features.
That part is not existent in 4E. Everyone uses the same resource management/power system. But they still have different class features, and different "spells". Some class features are even "hidden" in the system, like the Barbarians Rage powers. (Well, it's not really hidden if the barbarian description explicitly describes this...

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