Don't know my four number code

You can reset your ipad by shutting it off completely, then with itunes open on your computer, hold the home button down and plug in your ipad. Keep holding the home button until you get a message in itunes that will let you know your device is in recovery mode and must be restored. Next just follow the steps to restore your ipad. After the restore it will ask you re enter your apple id and password. Hopefully you have access to that. If you do, then you're home free!

I don't know why this happens, but some people report the same thing that you are experiencing - the iPad is requiring a passcode even though a passcode was never used on the device in the past. You can't get past it or find it somewhere. You have to restore the device now.

The advice posted above should work for you and allow you to get back into your iPad however, bear in mind that what that will do is totally erase the device and you will be starting over again. You can download most past purchases as long as you use the same Apple ID that you used to make the purchases. If you backup, you can restore from the backup after you unlock the device in order to recover all of your app data and device settings.

This will provide some more information for you. Read this all the way through including the very last part that talks about using recovery mode. You may need to use that in order to restore.