how to change passcode when I want to use 4 digits not 6 digits

Settings > [Touch/Face] ID & Passcode > Change Passcode > Passcode Options …

To change the length of your passcode, go to:

Settings > Touch ID & Passcode > Change Passcode.

Type in your current passcode again, then select "Password Options" and select 4-Digit Numeric Code.

Then write your new 4-digit code.