Does it take having to fly bombers and gvs?
To get 4 stars you basically gotta play everything, though not necessarily to the same degree. I went from 0 to 4 twice, but I was pretty much an all round type of player and (even more important) was putting insane hours into the game. Three stars should be able to be done in fighters only without any major problems, but you gotta spread out there, too - fly different planes of all nations and watch for those historical tasks and challenges.
