+99 on Kerlix/rolled gauze. I would rather have Kerlix and no 4x4s than the other way around. I can cut off a little Kerlix and make my own 4x4s.

For kits where space is at a premium try the PriMed compressed gauze from H&H. They take a full sized roll of Kerlix and vacuum pack it into a wad the size of slightly rotund golf ball. Even if the vacuum pack develops a leak its still less than 1/2 the bulk of uncompressed Kerlix. N.A. Rescue's s-packed gauze is similar but is vacuum packed in a "S" stack instead of rolled, which helps when feeding it out into a bad bleed.

4x4s are for cuts that are NBFD. Kerlix is for serious wounds. Cram Kerlix in there then top it off with a (possibly improvised) compression bandage. If the wound has a significant void cram in the Kerlix then add an object like a tennis ball, bag of fluid, etc and place the compression bandage over it. The idea is to transmit pressure to the bleeding site, not simply absorb the blood. Soft spongy stuff is not ideal as the medium for transmitting pressure.

Of course you're unlikely to have a tennis ball or bag of fluid on hand but that's the size & consistency of "filler material" we're talking about.