Find Longest Sequence of Consecutive Composite Numbers in Java

Write a Java program to display the longest sequence of consecutive composite numbers up to N. For example, if N = 10, then the longest sequence of consecutive composite numbers in this range would be 8, 9, 10.

import java.util.Scanner;
class Sequence{
    public static void main(String args[]){
        Scanner in = new Scanner(System.in);
        System.out.print("N = ");
        int n = Integer.parseInt(in.nextLine());
        int start = 4;
        if(n < start){
            System.out.println("n should at least be 4!");
            return;
        }
        int count = 0;
        int counter = 0;
        String s = "";
        String seq = "";
        for(int i = start; i <= n; i++){
            for(int j = i; j <= n; j++){
                if(!isPrime(j)){
                    count++;
                    if(count == 1)
                        s += j;
                    else
                        s += ", " + j;
                }
                else
                    break;
            }
            if(count > counter){
                counter = count;
                seq = new String(s);
            }
            count = 0;
            s = "";
        }
        System.out.println(seq);
    }
    public static boolean isPrime(int n){
        int f = 0;
        for(int i = 1; i <= n; i++){
            if(n % i == 0)
                f++;
        }
        return f == 2;
    }
}