El-ad David Amir
Is it always the same, we've only played two player, and even then we run out of buildings of one type or another usually. In the 4 player game is there just half as many buildings available?
While there is often competition for Barracks, and rarely for Trading Posts or Harbors, in my experience the other buildings rarely run out. Remember that the game is supposed to end by turn 10-12 (sometimes even earlier). Somewhere around turn 5-8, you probably want to stop spending your city actions on buildings and move on to actions that directly support winning the game (rather than buffing your infrastructure).