I believe I used to set mine at twenty five feet. Four feet seems a mite close for accurate readings as particles and smoke affect the readings. And never could get an accurate reading using black powder.
I go 10-12 feet also. Even at that distance the muzzle blast from .308/.30-06 class rifles sometimes gives me bad readings. Handguns have been no problem.
Using an Oehler 33 years ago for shotguns, I used 3 ft to the first screen. Handguns and .22s, about 5 ft. Centerfire rifles, 10 ft usually was far enough not to blow the readings. .30-06 is my biggest, too.
At least 10 feet. Smoke, unburned powder and particles can get you an incorrect reading if you are too close. I shot a 38 special smokeless powder BLANK (Winchester) thru my chrony at about 8 feet and it recorded SOMETHING at close to 1600 fps.