I'm planning to run a game or two at an upcoming convention, particularly meant to introduce new players to the game. But I've only ever played with 1 or 2 players, so I'm not sure what to go with as far as decks and scenarios. As I understand it, the scaling for some quests is not good, where 4p is far too easy. But I don't want it to be a super difficult challenge, either. I suppose I may just go with straight up single spheres for each player, so I don't have to worry too much about constructing 4 different decks that will work alright together.

But what would you recommend? I have 2 core sets, all APs (except for Massing) and Khazad-dum.

I do exactly what you said. 4 mono-sphere decks. I would use Journey Down the Anduin from the core set, that is a nice balanced quest.

If you want to introduce new players during a convention, just go with the 30 cards starter decks and the very first quest (Spiders of Mirkwood). There is quite a lot going on in the game, and getting to know the mechanics is difficult enough. Also, keep in mind that winning the first game will be a positive experience for the players and will make them more likely to come back. If you go with Journey down the Anduin, that Hill Troll will cause some serious trouble, and you will need to give a lot of advice to handle the situation. My experience with this game is that new players don't really like it when more experienced players give them too much advice.

By the way, this was the way I was introduced to the game, with 2 friends and a stranger as the fourth player during a small convention. We did win the quest rather easily, and the guy who taught us told us it would become a lot harder. My girlfriend and I were curious enough to buy the game, and well, the rest is history.