How do I change to a 4 digit passcode

Do you already have a passcode set up on the device? What version of the iPhone and iOS are you currently using? Your profile looks rather out of date with iOS 5.

Go to Settings>Touch ID & Passcode. Enter your current passcode. Scroll down and find Change Passcode. You will have to enter your current passcode again. This will bring you to a page to enter the new passcode, and if you look just above the number pad, you will see a link for Passcode Options. Tap that, and it will give you the options for the 4-digit or a custom set.

Go to Settings > Passcode (may say "Touch ID & Passcode if you have an iPhone 5S or later; you haven't told us which one you have) > Change Passcode. Enter your current passcode. Then, when the number keypad pops up, tap on "Passcode Options," and choose 4-Digit Numeric Code.

momchone wrote:

How do I set up a 4 digit passcode?

Go to Settings>Passcode [Touch ID & Passcode on newer devices]

When you are changing the passcode, tap Passcode Options.

IF you have a work related account setup on your device, you may not have the ability to select 4 digit for security reasons related to that work account.