Four inches is exaggerated. According to Rav Chaim Naeh a tefach is 8 centimeters. According to the Chazon Ish it is 9.6.This works out to either 3.15 or 3.78 inches. Some round the Chazon Ish’s measure to 10 centimeters, which is 3.94 inches.Thus, four inches is erva according to all opinions. As it is very difficult to be exacting it is probabaly best to round down to three inches.
