fourDigits method
Implementation
String fourDigits(int n) {
int absN = n.abs();
String sign = n < 0 ? "-" : "";
if (absN >= 1000) return "$n";
if (absN >= 100) return "${sign}0$absN";
if (absN >= 10) return "${sign}00$absN";
return "${sign}000$absN";
}