To celebrate the 20th anniversary edition of The Settlers of Catan, Mayfair has produced a 5th edition of the game, as well as its expansions and extensions. I finished completing my Catan collection early this year, so I consider myself fortunate that I have a nice complete set.
The following, from Mayfair:
The die lines and die cut remain identical, meaning you could mix and match a 4th copy of Settlers with a 5th edition copy of Seafarers without difficulty or compatibility issues. The card backs are similarly identical, allowing you to mix cards from different editions of the game. Similarly the rules remain consistent, though there was an extensive balance and readability pass, meaning numerous rules are clarified with input from designer Klaus Teuber and numerous rare "corner cases" in the rules are explained. These new clarified rules will become the official tournament canon and for those looking to brush up on the clarifications, the rules will be online in PDF form once 5th edition is released.
The game has SLIGHTLY updated rules, nothing too significant, as the game is compatible with the 4th edition. A (slightly) different visual style has been used as well, and it appears to be cohesive with the other new editions of the expansions / extensions. Also, the name has changed. The original base game WAS called The Settlers of Catan, but now it’s merely Catan. [also, the naming reflects my own naming conventions a bit. Because the original was properly called “The Settlers of Catan”, I took to using the name of the expansion I was playing as the name of the game. Example, I call the ‘Sefarers’ one “Seafarers of Catan” or, “Cities and Knights of Catan”.]
Even though the 4th and 5th editions seem to be, as far as I know, be compatible with one another, I strongly urge any future buyers to attempt to own a complete set of either one or the other. All the game pieces etc will match…it’s just “better”. [another self-thought: Explorers and Pirates is the most recent Catan expansion, so I don’t think there’s too much change (if any) in the 5th edition compared to the 4th of that expansion.]. That may not matter to some, but it ensues maximum compatibility.
I’m sure glad I managed to get a full matching set of all the expansions that can be used with another! [Explorers and Pirates can’t be mixed with the other expansions]. I would not be surprised if some retailers are still in the process of switching over from the older version to the newer, just check out the slightly updated box art for which ones you want to get. On the Catan shop site, they only have the new 5th editions to choose from, so that's the official definitive way to ensure you get the newest editions, if you're after the 5th editions.
This is among my favourite (non video) games, possibly, my favourite. I strongly urge you to check out the main, base game if you have not done so.
For those interested, you can find the 5th edition rules here: http://www.catan.com/game/catan#downloads
Cities and Knights: http://www.catan.com/game/catan-cities-knights-expansion#downloads
Traders and Barbarians: http://www.catan.com/game/catan-traders-barbarians-expansion#downloads
Explorers and Pirates: http://www.catan.com/game/catan-explorers-pirates-expansion#downloads
Also, check out the FAQs for each game.