public class Test {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder(4);
String str="1234567890abcdefghijklmnopqrstuvwxyz";
for (int i=0;i<4;i++){
char c = str.charAt((int) new Random().nextInt(str.length()));
sb.append(c);
}
System.out.println(sb);
}
}