The LIQUID app has its own 4-digit passcode, separate from your phone's authentication method.
In case you forget your passcode, you can simply reset it by clicking on Forgot Passcode? and then proceed with Unsync wallet and reset.
In order to prevent unwarranted access to your portfolio, resetting the passcode will also 'unsync' the app. This means that all your portfolio data will be removed from your phone.
After setting up a new 4-digit passcode, you will need to synchronise with your ZERO again.