As two members mentioned I timed 4f and 3f as primer. The difference is measurable, but not with human senses. I like 4f, but 3f works fine. I'd rather use 3f than grind up some for primer.
Some shooters like priming from the horn rather than carrying an extra priming horn. For me it's the opposite. I have nerve damage in my hand and do not handle small items well. I hate handling the horn and the stopper twice. My flat priming horn is in a side pocket. It has no stopper, so it is easier for me than handling the horn stopper twice. If I ran out of 4f, I probably try 3f in my priming horn.
I'd suggest keeping the vent sqeeeeky clean, sharp flint, and making sure that there is priming against the barrel. These things can be as important.
Regards,
Pletch