Passcode

Is this your corporate iPhone? Do you have any profile installed by your corporate / organisation / enterprise / School Management then please contact the system administrator of your corporate / organisation / enterprise / School Management

If you have installed it on your own, remove it--> Install or remove configuration profiles on iPhone - Apple ...

Once you delete the profile, you should be able to change the passcode and set it to 4 digits again in Settings->Touch ID/Face ID & Passcodes->Change Passcode, then tap on the "Passcode Options" text to select a new type of passcode.

I'm confused now about what what you want to avoid.

You wanted to change it to a 4 digit passcode, if so, then you are going to to change it. You cannot convert a 6 digit passcode into a 4 digit passcode. It needs to be changed to 4 digits. Even if its the first 4 digits of the 6 digit one you had.

If you mean avoiding the need for 6 digit passcode, then contact your office IT department as they are the ones that set that requirement through the Exchange profile that is installed when you setup your outlook email.

They are the ones requiring this, not your iPhone or even Apple.