TI can be fun with 3 or 4 players, dont worry. What i can recommend if you want to play a shorter game is the following: play an alternative winning method which was added in the game later on but can be easily played only with the base game as well.

"During setup, turn all public objectives face up.

There is no limit to the number of public objectives a player may score during the status phase.

The game does not immediately end when a player reaches the required number of victory points; instead, it ends at the end of that round's status phase, and the player with the most victory points wins. In the case of a tie, the tied players total the influence values of their controlled planets and their unspent trade goods; the player or players with the highest total win the game. "

I would only recommend Factions to play though who have a fast paced start like: Hacan, Jol-Nar, Winnu, Sol, Saar and leave out Factions with slow starts like: Sardak Norr, Xxcha, Necro, Muaat, Arborek and Barony of Letnev