how to continue run the java programme

Since you did not write how you defined keyboard, I assume
keyboard is:
import java.io.*;
....

BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));

You need to catch IOException when reading line:
In order to match the content of input_4 with another string, you need to do this instead:
try {
String input_4 = keyboard.readLine();
if (input_4.equals("y"))
System.out.println("you want to continue!");
else
System.out.println("Thank you!");
} catch (IOException e) {
// Exception handler
}