I cannot find the 4 digits passcode option in my iPhone 12 Pro

Try this

Settings > FaceID & Passcode > Change Passcode > Enter a New 6 digit Passcode once [Only Once] > Tap "Passcode Options" in blue at the bottom [You will see the 4 Digit Passocde option here]

If you're talking about the passcode to lock/unlock the screen of your 12, it should be in Settings>Face ID & Passcode>Passcode options... as per this paragraph:

Change Passcode: Enter a new six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code.

Extracted from:--> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio