Its an IOS thing, Apple are requiring the passcode to be 6 digits.
We require 8 digits here, and when setting up we have to choose custom in the IOS passcode options on the device, to get it to allow the extra 2 digits. But I don't think you can use these options to set it to 4 digits.
On the IOS device choose to set the passcode, and the 6 digit box will pop up, in it will be custom options that you can choose.
Paul, thanks for your replay. Nice to know because I started to think thats this is something related to Jamf, possible due to configuration profile.
@jamflund Nothing wrong with your profile, or JAMF. We actually use a different MDM for our IOS and Android devices, and it is the same there too. (I would use JAMF, but we need to cover Android devices)
I think you may find in the future that this complexity will increase. We are forced to go to 8 digits by security accreditation that we are signed up to.
Glad to be a help