Although for security reasons, when you have set up Touch ID or apple pay options, you cannot turn off this option.

Allow Access when locked
This option lets you access certain information like the Today view and the notification centre when your screen is locked. When you think are no critical data contained in these locations and you wish to access them quickly, you can consider turning these options on.
Erase Data
This is another cool feature in terms of security. When someone (even you) tries to enter an incorrect passcode 10 consecutive times, this setting erases the device automatically. This can help you to make sure that no one can brute force their way in by guessing your passcodes. Although even when you have not enabled this option, the phone gets locked completely after ten unsuccessful attempts. But you can restore the phone using a computer later.
Passcode For Business Devices
If you are using a device provided by your organization for business purposes, then you will not be able to change most of these settings because such business devices are controlled by passcode policies governed by your organization. In this case, there is nothing you could do except to ask for help from the IT administrator of your organization. They should have control over your device. here are the things you’ll face if you have a business device that has company policies:
- You’ll not be able to turn off or change your passcode.
- You’ll not be able to change the passcode because they might be disabled or greyed out.
- You may get notifications as your password has expired.

In this situation, you should contact your business HR or IT admin to solve this issue for you.
Conclusion
So this is how you can switch to a four-digit passcode option on your iPhone or iPad even though the six-digit passcode is much secure than the four-digit code because someone could also be able to guess this. Because most of the time, when people set up a f our-digit code, they mostly enter their birth year or birth date. One could easily guess this. So if you want to opt for a four-digit code for the sake of convenience, then you should at least try to enter something that someone you know might not guess easily.