Dungeons & Dragons Fourth Edition, often abbreviated as just "Fourth Edition" or even "4e", was the 2007 revision of the classic roleplaying game system that inspired the Order of the Stick strip.
4e proved to be fairly controversial and not quite as commercially succesful as Wizards of the Coast hoped it to be. A Fifth Edition eventually followed it, being released in 2014, at a time when the Fourth Edition had been largely abandoned and the Pathfinder system had inherited much of the D&D market.
Despite an opening strip that revolves around the conversion of the party from D&D 3.0 to D&D 3.5, Rich Burlew decided against converting the characters to 4e, nor later to 5e. By the time 4e was released the strip was well into its movement towards less emphasis on rule system jokes and into stronger plot and characterization, and system-specific jokes or other references have become few and far between.
There are very few references to Fourth Edition in the comics. Even so, Kubota is seen reading the recently-released Dungeon Master's Guide of 4e in #562 and the main feature of Snips, Snails, and Dragon Tales is a meeting between the characters of the Dragon magazine continuity with their 4e counterparts.