I have the English fourth edition 2007 base game of Settlers of Catan. Are current editions of the expansion, specifically Catan: Seafarers English edition 2015, compatible with this base set?
When the 5th edition of Catan came out, I remember reading an official source (Catan FAQ) explaining that it would be completely backwards compatible. The only source I can find right now is this one:
The 5th edition will also be fully backwards compatible to the previous 4th edition and will feature a more cohesive art style throughout all 4th and 5th edition expansions.
It looks a bit odd though, using different editions. So at that time I decided to buy all the old sets while they were still available. It's functionally possible to combine different editions into 1 game, but not aesthetically correct :)