no, by default it set to 6 digits, but you can choose to have 4.
My device running iOS 13.3, my wife is running on a 4 digits since iOS9
it's french, but allow me to choose 4 digits passcode.
I just update an old iphone7 ready to give to a new employee, the old device was off for +6 months, when I open it, the 180 day change code policy ask my to change it, it was 4 and only 6 was possible now. It still run iOS 12.x
if I remove the passcode policy settings from the phone, I can set it to 4... so definitely it has to do with this MDM settings pushing something wrong!
I was with another MDM supplier before, never had this problem.