BufferedReader br = new BufferedReader(new FileReader("test.txt"));
PrintStream ps = new PrintStream(new FileOutputStream("test.txt.new"));
String buf;
int count = 0;
while ((buf=br.readLine()) != null) {
    count++;
    if (count == 4) {
        buf = buf.replaceAll("xxx", "yyy");
    }
    ps.println(buf);
}
br.close();
ps.close();
File f1 = new File("test.txt");
f1.delete();
File f2 = new File("test.txt.new");
f2.renameTo(f1);