Brain tanned would be too soft I guess. Guys and gals here use pig brains mostly, for tanning their clothing leathers for rendezvous. It breathes, whereas chrome (normal) tanning doesn't.

I do not know what Swiss 4 is. I would be using Swiss 2F at the very finest, but some make of 4F for priming powder only.
I use 4F for prime in all my flinters, rifle and shotgun.

Before I attempted any shooting, I would pull the breech plugs and check the bores. It looks super on the outside. It would be a shame to damage it.

Also, is it loaded?

Many guns, rifles, handguns and shotguns were stored loaded and never unloaded or fired after putting away, the last time.