momsworry,
Like most of these guys say, it's what works best for you. Another forum online is frequented by many of the best gun-part builders. One of those is Jim Chambers, from whom many of us have purchased our locks. Chambers once posted a response to the very question you ask pointing out, unequivocally, that none of the powders burn any faster than any other. In fact, as has been said, 4f is probably the least effective prime powder due to the lack of any protection between powder and moisture. Chambers claimed to have measured the various grades of powder with a high-speed camera and computer. According to his research, no powder, fine or coarse, ignited even a fraction of a second faster than any other.
In my own experience I began, as most of us, under the misconception that 4f was the only powder to use in a pan. After hunting in rain and snow countless times since I have come to the point where I only use 3f in the pan. I have had several occasions when, carelessly, I let snow or moisture reach my lock. Even after all attempts at drying the mechanism I was still left with mis-fires using 4f. However, immediately upon applying 3f the gun fired, in every case, without hesitation.