Apple ID Password requirements
There are two types of passwords, the device passcode and the appleid password.

The device passcode gives you access to unlock the device, and it can be as short as a 4 digit PIN.

The AppleID password must be 9 (I think. It's either says "8" or "more than 8") or more characters long, contain at least one capital, one lowercase, and use at least one number or special character.