Level 4 - Level 5

Username: natas5
Password: Z0NsrtIkJoKALBCLi5eqFfcRN82Au2oD
URL:      http://natas5.natas.labs.overthewire.org

This time, the responded that "You are not logged in".

So I checked the HTTP history of burpsuite and found a cookie named loggedin in the response header, with a value of 0.

I tried modifying the value to 1 using the Dev-Tools.

And refreshed the site and got the password for the next level.