King Arthur Pendragon is a roleplaying game set in Arthurian Britain. The game's vision of Arthurian Britain influenced by literature from Dark Ages legend through to modern interpretations, but the literature of the high Middle Ages is most influential. The game system is built for this version of Arthurian Britain, with the players all playing knights. Perhaps because of this focus, the game system is as close to perfect for the setting and narrative style of the game as I have ever found in any roleplaying game.
The core of the game has been almost unchanged through its various editions - if you purchase Pendragon 1st Edition you'll see how little the core mechanics have changed throughout the editions. The key difference between 5th edition and 4th edition is that 5th edition is more strongly focussed on the players playing knights, and that 5th edition is strongly coupled with "The Great Pendragon Campaign", which the Gamesmaster can use to guide the player knights through the entire Arthurian legend, from Arthur's conception through to the Battle of Camlann. The feel of 5th edition is therefore a lot more true to the literature and game designer's original intention than 4th edition. If, however, your group wants to play magicians, then 4th edition may be a better purchase as it includes magic rules. Both books will be equally compatible with the various supplements - you'll just have to work harder with 4th edition to play "The Great Pendragon Campaign".