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I haven't played Settlers of Catan yet, but I've been considering asking my gaming group if they want to try it.
As I understand it, Settlers of Catan is now in its fourth major version. If I do get a Catan box, should I just get the current (fourth) edition, or is there some reason to look for used copies of earlier editions?
Edit: I forgot to mention, we will also be getting the 5-6 player extension.
The new fourth edition has a few minor changes in appearance, and the biggest change is a new set of border pieces that surround the hexagon tiles to keep them together.
The most important thing to know is that they do not mix and match very well: if you have the 3rd edition of Settlers, you should buy the 3rd edition of the expansions. If you have the 4th edition of the base game, you should buy the 4th edition expansions. So if you're just getting into it, I would suggest buying the new edition, since the corresponding expansions will be easier to find.
In Cities & Knights, there changed what some of the progress cards are. The card that lets you disable an opponents city has been removed in the 4th edition. I'm not sure what additions it was in, but one of my friends complained quite loudly when he realized they had removed the card.
I would get the 4th edition, since it's easier to find, and the expansions will be easier to get. Most of the changes have been minor and make game play easier. My only complaint about the quality of the 4th edition is that I wish the frame pieces were sturdier.
There was a huge (and stupid) rule addition with the later editions, where if you run out of resource cards during a production phase, no one gets anything. In the old rules (1999, Mayfair) there was no rule on it, so our group always played where you parcel out that resource starting at the die roller, and going clockwise.
The new rules also allow you to move the robber to a desert hex, which was not allowed before.
I own the German version (Die Siedler von Catan), and it has different (plastic) pieces for roads, etc. The board pieces are a slightly different size, as well as the resource cards. Therefore, if you plan on getting the expansions, make sure that you stick with the US versions.
I would recommend the 5th edition if you're getting started, as another person here commented - it will be much easier to find expansions. For earlier editions you're essentially buying collector's items and in some cases could pay much more for the expansions than a brand new 5e. The 3e hexes are slightly bigger (79.375mm) compared to the 4e and 5e hexes (78.5mm). There is no frame for the 3e in the box, but there is a "yucatan board" that holds the basic and 5-6 player expansion quite nicely. It can be usually be found on eBay.