I need help for my Java Assigment , replacing string

Hi , I have a assigment that was already due but I still want to work on it.
For this program im trying to replace every 4 words with with an astericks (*). It is suppose to be read from a file
and the line cannot exceed more than 80 characters.
So this what I have so far...

I'm also not allowed to use java codes that are off my level because I am new at this.

Code Java:

/**
 * 
 */
package replace;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
 
public class replace {
 
 
	public static void main(String[] args) throws IOException {
 
		File file = new File ("input.txt");
 
		Scanner inputFile= new Scanner(file);
 
		String input;
		String line;
		int StringLength;
 
		line=inputFile.nextLine(); //read first line
 
	for(int x=0;x<line;x++){
		line=inputFile.nextLine();//read line
		StringLength=line.length(); //get length of string
		int y;
		while( y<StringLength){
 
			if (input.charAt(y)isWhiteSpace); //save the characters before whitespace
 
			System.out.println(y);
		}
 
 
	}
 
 
 
 
 
	}
 
	}
 
}