I've been working on a card game where cards are played either for points or used for their special abilities. The game runs really smoothly with 2 people and is very "tight." I've gotten feedback though that a 2-player only game is harder to sell than a 2-4 player game. having the 3 and 4 player game follow the same rules as the 2 player game has not worked well. The importance of a player's decisions is lost with so many other players' turns happening between his or her turns. I solved this by making a 4-player version that involves 2 teams, each holding half of the 2-player hand size and teamates having limited knowledge of their partner's cards. This also is a fun game, and I enjoy it, but it really is more like a variation of the 2-player game than it is like a scaling up of the 2-player game. The 3-player game involves a team playing against an individual, and requires a tweak to the scoring to balance the team against the individual. The 3-player game is not "broken" but it is weak. In summary I can imagine a reviewer in the future writing "The 2-player game is great, and the 4-player variation is also pretty good, but I would not recommend playing the 3-player variation."
My greatest fear is that if I put "players: 2-4" on the side of the box, that someone will play the 3-player version, find it meh or awkward, and never play my game again. What I really want is people to judge it on the 2-player experience, and then find the 4-player experience to be a neat variation that gives some flexibility to player count.
My real question, and maybe you can't answer it for me, is whether I market the game as a 2 player game and include rules for a 4 player variation, market it as a 2 or 4 players game (is that odd "2 or 4"?), or market it as a 2-4 player game and hope that the word gets out that it is "recommended with 2 players"?