These days, I carry a 4" folder everywhere I go, unless I'm carrying a larger knife. I wouldn't hesitate to use it for self-defense if it was all I had, but I would never PLAN for it to be my primary self-defense weapon.I find a 4" folder easier to carry than a pistol.
I agree that a 4" folder is easier to carry than a gun. But I don't consider a 4" folder to be a "large fighting knife". Any knife I would consider using as a primary self-defense weapon would be much larger than a 4" folder and harder to carry than a full-sized duty pistol.
Yes, firearms are loud. I have pretty bad hearing damage in my left ear as a result of firing a .357Mag inside without hearing protection. It was a bad experience and every day, I regret the damage it did, but I'd do it again in a heartbeat rather than choose to face an assailant with a medium-sized folder as my primary defensive weapon.
I seriously hope that there aren't many people out there who could carry a pistol for self-defense and are instead relying on a medium-sized folding knife as a primary self-defense weapon.