There's certainly nothing dangerous about your load. It's only important for the powder, wads, and shot to fill plastic and paper shells so that they crimp shut properly. If you are just going to use an overshot wad to close the shell, then it's not really important to fill the case.
One cc of black powder weighs about 13.6 grains so 4.3 cc = about 58 grains, that's a pretty light 12 gauge load but if it does the job, no problem.
My black powder trap load is 70 grains of FFFg Goex black powder, two .135 nitro cards, a 1/2 inch fiber wad, and 1 1/8 ounce of shot loaded in a plastic Winchester AA case. That takes up just enough room to allow a good star crimp. If the case is too full, you can't close the crimp all the way, if there is too little stuff in the case, the crimp folds inward and shot spills out through the center.
But since brass shells aren't star crimped, it doesn't matter if the powder, wad, and shot don't completely fill the case.