I would proceed to buy a can of 3-FFF. Once gotten make your own 4-FFFF by chopping up 3-FFF into smaller pieces on a plastic cutting board or a small piece of flat glass which works best. 2-3-4-FFFF are all made of the same mixture. The only difference between them is the size of their kernel/ grain.
I once drove up to my cabin to B/P deer hunt. But left my 4-FFFF Gorex can home. 253 miles I would have to drive back to get it. But I did have a horn of 2-FF along. I drove to the closes big town and all I could find at the time was Pyrodex RS. Disappointed and back at the cabin I though about that long ride back home and the lost of two days vacation over a simple mistake on my part. Low and behold an old timer who was a good friend of my fathers stopped by in the early afternoon for a meet and greet and listened to my tail of woe over a cup of coffee. He's the one who told me to chop up some 2-FF into a size similar looking to 4-FFFF with a single edge Stanley saftey razor blade. I performed the task on a tea cup saucer. Not much of it but enough to prime my rifles pan at least 2-3 times. First little bit I chopped I immediately primed my rifles pan right there in the kitchen and by George it work slick on the first hammer fall. By the following days evening hunt I had a nice spike laying down on the ground in front of my stand 50-60 ft away. I've only bought (1) can of Gorex 4-FFFF in 25 or 30 years. And to this day it hasn't been opened more than once with only a once or so removed its entire time here. Been chopping up 2 and 3-FFF into 4-FFFF ever since that day in my cabins kitchen. That old timer is long gone now. But that little bit of help and knowledge he passed on to me still lingers.