Overview
Dungeons & Dragons 4th Edition is a tactical, combat-focused tabletop roleplaying system released by Wizards of the Coast in 2008. Designed to fix the high-level math imbalances and "martial vs. caster" disparities of the 3.5E era, 4E redesigned the game's engine with a more video game adjacent approach. It is famous for its structured, grid-based combat and for standardizing class abilities into a unified system. While its dramatic departure from legacy mechanics polarized the fanbase directly paving the way for the creation of
Pathfinder 
https://groupfinder.eu/library/pathfinder-2009 . 4E is widely praised today for its brilliant monster design, DM tools, and unparalleled encounter balance.
The system it directly replaced is
Dungeons & Dragons 3.5 Edition (2003)
Description
In D&D 4E, players take on the roles of heroes who are combat-capable right from the start. The system relies of combat being carried out with the use of a battle grid. Positioning plays a big part of the combat experience. The exact location of the characters matters greatly.
Unlike other editions where spellcasters are played in a separate mechanical way than martial classes, 4E unifies the progression system. Every class is boiled down to a specific combat "Role" and draws from their specialized toolkit of "Powers," making the game balanced and ensuring that every character has unique and impactful actions that they can perform on their turns.
System Overview & Key Features
The Power System
Instead of traditional spell slots or basic attacks, every class has a list of At-Will, Encounter, Daily, and Utility powers, each with different power levels and frequency at which they can be performed. Fighters can execute epic daily maneuvers just as wizards cast daily spells, creating a better balance between martial classes and spellcasters.
Combat Roles
All classes are sorted by their class specialization role: Defenders (tanks who draw aggro and protect their allies), Strikers (high single-target damage), Leaders (healers and support), and Controllers (area-of-effect damage and debuffers).
Static Defenses
Saving throws were removed and replaced with four different defense values: Armor Class, Fortitude, Reflex, and Will. When a wizard casts a fireball, they roll an attack against the targets' Reflex. Instead of both sides rolling for success and for defense - the attacker is the only one to roll dice.
Healing Surges
A standardized resource pool representing a character's stamina. Almost all healing in the game, whether from a cleric's spell or catching your breath, requires the target to spend a Healing Surge, which reliably heals them for exactly one-quarter of their maximum hit points.
Monster Roles & Minions
The Dungeon Master's toolkit was revolutionized. Monsters have explicit roles (like Brute, Artillery, or Skirmisher) that cleanly dictate their behavior. 4E also introduced "Minions". Monsters that have normal stats and deal average damage, but only have exactly 1 Hit Point, allowing heroes to cleave through cinematic hordes.
Skill Challenges
An unique system with 4E is Skill Challenges - a "cinematic" series of actions that turn social, environmental or escape situations into more movie-like scenes. The players must achieve a certain amount (determined by the situation) of successful skill checks, before they collect enough failed skill checks.
Additional links
dungeonsanddragons.com https://dungeonsanddragons.com - Official Dungeons & Dragons website