public class Test {

  static boolean check(String str) {
    int pos = 0;
    while (true) {
      if (pos == str.length()) {
        return false;
      }
      char c = str.charAt(pos);
      if (c == '0') {
        pos++;
        continue;
      }
      return (c == '4');
    }
  }

  public static void main(String[] args) {

    System.out.println(check("123456")); //false
    System.out.println(check("423456")); //true
    System.out.println(check("0043456"));//true
    System.out.println(check("123456"));//false
    System.out.println(check("0123456"));//false
    System.out.println(check("4"));//true
    System.out.println(check("0"));//false
    System.out.println(check(""));//false
  }
}