Some time ago, before I started using a borescope, I did test H9 vs C4.
I first pushed a couple patches with H9 through the bore, brushed with a bronze brush 20 strokes, patched out with H9 and dry patches until the dry patches were essentially clean.
Then pushed a patch with C4 through - a bunch of black came out => no question C4 works better.
Once I started using a borescope, I discovered that even with using C4, there is still a LOT of carbon left in the bore. So much, that I doubt the difference between H9 and C4 makes any difference.
For example, I see no visible difference in the amount of carbon in the barrel after I use C4 and a bronze brush and one dry patch vs continuing on using wet/dry patches until the dry patches come out clean.