Mandatory Combined InputField

I've got a combined input field "Geo Hierarchy". This field itself isn't mandatory. It contains 4 inputs: "Region", "SubRegion", "Area", "SubArea". Then the user starts filling this input only the first 3 inputs are mandatory, the last one is optional. Could it be shown visually that the last input is optional?

Now we're marking fields as mandatory using dots inside the field.

Proposal: to add mandatory markers to the appropriate fields. The proposal was rejected for this reason: Fields don't have mandatory markers that stand near their titles and if the field is mandatory this dot will be shown immediately (like in the example of Distance Catalogue Input). But discussed field Geo Hierarchy isn't mandatory itself.