Assuming Blabla has the fields in your question, as well as field4,
Blabla.objects.only('field1', 'field2', 'field3')[0].field4
will return the value of that object’s field4 (with a new database query to retrieve that info), whereas
Blabla.objects.values('field1', 'field2', 'field3')[0].field4
will give
AttributeError: 'dict' object has no attribute 'field4'
This is because .values() returns a QuerySet that returns dictionaries, which is essentially a list of dicts, rather than model instances (Blabla).