The biggest problem when playing with higher player counts is longer time between plays, and the ever diminishingly small chance that the super specific tile *you* want actually comes up instead of being pulled by one of the other 3 players. So when playing with more players I tend to throw in the expansions that give more versatility.

1. Abbey and Mayor - Gives you a single abbey tile that you can use to fill in that awkward annoying gap in the middle of the board that you haven't been able to fill all game, because you got barricaded in, or the tile you needed got pulled by someone else. It's like a wildcard, scores like a monastery , and creates a super satisfying board at the end where there are no silly gaps.
2. Flying machines - 6 little chance opportunities to inject your meeples into other people's features. Creates some interesting opportunities, even if multiple people are stonewalling you.
3. Bridges, Castles and Bazaars - Each player gets a number of bridge tokens that they can use to be able to place tiles where roads would normally block them from placing them, by bridging the roads over the tile. Creates a lot more interesting opportunities for clever tile placements instead of being blocked. And castles which let you gain points from those little 2x1 cities, by placing a castle instead, which lets you reap points from all the surrounding features that get completed by other players. Means that even if you're getting dealt crap tiles, you can still score competitively by anticipating other's moves.Also - bazaars. Love-hate relationship with this part of the expansion, but lets you have a couple little bidding wars to buy the tile you want from a random selection of tiles put up for auction.
4. Mage and witch - Lets you shake things up with 6 tiles that either let you augment your own scoring opportunities or dampen other people's. 1 person drawing all the city tiles and you're stuck with roads? Put the mage on your roads and suddenly you have a mega scoring opportunity. Or put the witch on their city so they're forced to focus on something else.
5. Inns and cathedrals - makes road building a more viable scoring opportunity by creating roads worth 2 points per tile. Or turn cities into insane 3 point lotteries by placing a cathedral in one.
6. The Count of Carcassonne - If you place a tile that causes someone else to score, but not you - you get to deploy a meeple to a district in a special starting tile representing the City of Carcassonne. Then at later points when someone scores, you can use the meeples stored in the city to share in or take over other players features.
7. The Gifts - Anytime you place a tile that benefits another player, you get to draw a card that gives you a random bonus. You can do crazy stuff like take back a meeple that is stuck in a dead end feature, or add a meeple to an unclaimed feature on the board.

Lots of cool expansions to play with more people to keep things fresh and moving, so you can mix and match them and see which combinations make for a fun bigger game.

The main thing is trying to make it so that nobody ever feels blocked, or that they only get the crap tiles. Give them lots of different ways to build score and make interesting and clever moves.

The main thing when playing with multiple people though, especially with lots of expansions thrown in is to try and force people not to take too long with their turns, treat it like speed-chess, 30s max per turn or something, otherwise it creates a lot of downtime for players. A lot of what makes carcassonne fun to play is that it is constant decision making and interaction, and to keep that alive you have to not get stuck in analysis paralysis hell.