Catan : how to play with extension 5-6 players when we are less than 5?
The 5-6 player extentions contains tiles, number fiches and additional playing pieces so there is enough to play with more players.

It contains scenario's for 5 and 6 players. 

You can play with 3 or 4 players on a 5 or 6 player board, but then the game becomes too easy. The board is just to big (contains too much resources) to have a challenging game. And yes, you can increase the number of VP needed, but keep in mind the game pieces max out at 13 VP, there are a few cards with extra VP and there is 4 points to collect with the longest road and the largest army. But keep in mind that if you need all these, the game is tied if two players buy a +1 VP card.

Of course, you can use pieces of the extention in the 3-4 games. But then you leave some other pieces out.