I have been a flintlocker for over 40 years. To answer the question: No. You do not NEED 4Fg to prime the pan. Some shooters use 1 or 3 Fg. for priming. Most prefer 4Fg. I prefer 4Fg.
A never ending debate among flintlockers is whether 4Fg is faster than the others. I cannot answer that definitively.
But, I will say I can detect slower ignition times using 2Fg. I cannot detect a difference between 3Fg and 4Fg. But human ears and reaction times are not perfect machines.
I am of the opinion 4Fg is faster and I use it.
Others opinions may vary.