My Lenovo A1000 one day suddenly got a password that I hadn't put on it

Added (1). It's 4 digits and after some attempts and a lot of luck I managed to unlock my phone. The problem is I can't remove the password in any way. I've gone through the settings and even set a new password but that just makes two passwords.

Added (2). If it isn't clear enough, my question is how do I remove this password