A well tuned flintlock will fire just fine with 2f for priming. It will fire quicker with 3f and even quicker with 4f, but with my experience, only by fractions of a second. For some shooters, that fraction can lasts moments, but for steady patient shooters, there isn't really much of a difference. At least, not to me. I do use the 4f for priming though because I do prefer ignition that is instantaneous.
graf and sons will ship you some 4f with no minimum purchase as of the last time i ordered from them.