If I purchase the Settlers of Catan 5-6 Expansion, is there a playable version of the game for only 4 people?
The 5-6 player expansion adds new hexes, ports, number chits and development cards to allow your board to fit 5 or 6 players. While you could play 3 or 4 players on the 5-6 player board, it would not be the same experience, you will find every one is too spread out and most players can expand unopposed.
Keeping the additional pieces mixed in with your base set may throw off the balance of the game. So it's recommended to remove them when playing a 3 or 4 player game. I don't remember the exact mix, but I believe the expansion adds the following
- 2 hexes of each basic type (forest/plains/mountains/fields/hills)
- 1 desert hex
- 1 sheep port
- 1 3:1 port
- 6 soldier cards
- 1 year of plenty card
- 1 monopoly card
- 1 road building card
- number chits
- 5 extra resources of each type.
The extra resources rarely becomes an issue in smaller games. It's probably safe to leave them in, and there's no harm and using one of the expansion sets of player pieces in place of one of the basic ones. But the rest of the items can definitely throw off the balance of the game.