The new iOS enforces a 6 digit password, but should allow you to keep your old 4 digit one if you upgrade, did it prompt you to use a 6 digit one after upgrade?
If you can't remember the old one, the only way to get in to it would be to wipe it from Find My iPhone/ICloud. or put the wrong password in for the 10th (or could be 15 trys) time, then it should wipe itself if that is enabled.