Hopefully you have a passcode set up on your iPhone, which is the easiest way to protect your iPhone if it's lost or stolen. Apple recommends setting up a 6-digit passcode for maximum protection. However, many people find it easier to entier a 4-digit passcode, similiar to a pin code.
Apple still provides the 4-digit passcode option, however, it's not that easy to find. Here's how to set up a 4-digit passcode on your iPhone or iPad:
Go to Settings > Touch ID & Passcode > Enter your Passcode > Select Change Passcode
You'll have to enter your old Passcode, and when asked to enter your new passcode, select Passcode Options:
Selet the option for 4-digit alphanumeric passcode.