Saying the 4th ed. is "incompatible" with the 3rd ed. is an overstatement. I have 3rd ed. "settlers" and 4th ed. "seafarers". They work together just fine if you can get over a couple of minor things:
1) the graphics on the hex cards are different, but they are still the same resources. To me this isn't a big deal, a wheat is a wheat; on my board with mixed editions, some wheat hexes (as well as all the other resources) look different from others (so what?). If you must have all your resource hexes look identical, then you'll want to make sure you get the same editions. I actually like them looking a bit different, so that I can quickly separate out the "seafarers" hexes from the "regular settlers" hexes.
2) the 4th ed. of "Settlers" comes with 6 border tiles that take the place of the old sea tiles in 3rd ed. "Settlers". The border tiles serve to hold the assembled hex pieces together a bit better, but certainly aren't necessary. "Seafarers" 4th ed. comes with a few extra border pieces to expand the border pieces of 4th ed. "Settlers". I don't have the "Settlers" border pieces with my 3rd ed., but you don't need to use the border pieces at all anyway. You can purchase the 6 border pieces to "upgrade" a 3rd Ed. "Settlers" separately, but this seems totally unnecessary to me since I've never really felt like the assembled hex board needed anything additional to keep it together.
3) the language of the resoures and hex tiles has changed. The wood resource and the hex tile that produces it are variably called "wood", "trees", "forest", or "lumber", in different game editions and by different players. The rock resource is similarly called "stone", "ore", "mountains", "iron", or "rock". The same is true also of "wheat/grain/fields", "bricks/hills/", and "sheep/wool/pasture". You can easily get over this if you are at all clever, but it can lead to some momentary confusion during the game if you your fellow players call the resources different names.
4) the development cards with soldiers in the 3rd ed. are called knights in the 4th ed. So "Seafarers" 4th ed. rules will refer to "knight cards", and these simply refer to the soldier cards from the previous edition.