Apps themselves should not have any control on the passcode length. If you have an Exchange email account or an office profile installed, it may require a specific length via a profile in Settings ➜ General ➜ VPN and Device Management.
You may not be able to remove it, if you still need access to the related Email account.
In any case, if you are now allowed to change the length, you can go to Settings ➜ Face ID & Passcode ➜ Change Passcode ➜ Passcode Options and select the 4-Digit Passcode option from the menu if available.
If it does not appear there, then you may still have a profile or email account determining the passcode length.