Thanks Alexandre, It looks like the id of the forms always stay the same and the id of field always stays the same, is that correct?
If so then we can do something like this?
form#formAcym11081 #field_4
It would be better if there was a way to add a class to a CF, so an admin who does not know CSS can just add a class of "hidden" to any field they want. I'll add it to the Feature Requests.